Gas sensor, and method for operating the gas sensor

ABSTRACT

Gas sensor, including a membrane and a heating element arranged on the membrane between a first discontinuation area of the membrane and a second discontinuation area of the membrane. The first discontinuation area of the membrane includes at least one discontinuation of the membrane and the second discontinuation area of the membrane includes at least one discontinuation of the membrane. The gas sensor further includes a first temperature sensor structure arranged at least partially on the membrane on a side of the first discontinuation area of the membrane opposite to the heating element, and a second temperature sensor structure arranged at least partially on the membrane on a side of the second discontinuation area of the membrane opposite to the heating element.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of copending InternationalApplication No. PCT/EP2019/050267, filed Jan. 7, 2019, which isincorporated herein by reference in its entirety, and additionallyclaims priority from European Application Nos. EP 18 150 493.7, filedJan. 5, 2018, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Embodiments according to the invention relate to a gas sensor and amethod for operating the gas sensor.

Currently, gases may be analyzed with respect to their properties usingdifferent sensors. Today, there are different systems for patientventilation on the market. They are distinguished according to theirutilization in the clinical area and in the home care area (e.g. systemsof the companies Heinen+Löwenstein, Dräger and Stephan Medizintechnik).The systems of these providers contain only in their top variations allmeasuring means for determining pressure, expiratory/inspiratory flow,and breathing gas analysis. To this end, several devices thatoverwhelmingly measure remotely from the patient have to be combined.

In light of the aforementioned, there is a need for a concept thatenables a better compromise between a reduction of an installation spaceand a reduction of a system weight of a gas measuring system, andprovides an exact flow measurement as well as a quick gas analysis.

SUMMARY

According to an embodiment, a gas sensor may have: a membrane; a heatingelement arranged on the membrane between a first discontinuation area ofthe membrane and a second discontinuation area of the membrane, whereinthe first discontinuation area of the membrane has at least onediscontinuation of the membrane, and wherein the second discontinuationarea of the membrane has at least one discontinuation of the membrane, afirst temperature sensor structure arranged at least partially on themembrane on a side of the first discontinuation area of the membraneopposite to the heating element; and a second temperature sensorstructure arranged at least partially on the membrane on a side of thesecond discontinuation area of the membrane opposite to the heatingelement.

According to another embodiment, a method for operating a gas sensor mayhave the steps of: heating a heating element; conducting heat via a gasmixture, wherein more heat is conducted from the heating element to atemperature sensor structure via the gas mixture surrounding the gassensor than via a membrane; and detecting a heating transfer by means ofthe hot ends of a temperature sensor structure.

According to another embodiment, a gas sensor may have: a membrane; aheating element arranged on the membrane between a first discontinuationarea of the membrane and a second discontinuation area of the membrane,wherein the first discontinuation area of the membrane has at least onediscontinuation of the membrane, and wherein the second discontinuationarea of the membrane has at least one discontinuation of the membrane, afirst thermal element structure having a hot end arranged on themembrane on a side of the first discontinuation area of the membraneopposite to the heating element; and a second thermal element structurehaving a hot end arranged on the membrane on a side of the seconddiscontinuation area of the membrane opposite to the heating element.

An embodiment concerns a gas sensor (e.g. in the form of a MEMScomponent) including a membrane (e.g. a thin-layer membrane whosethickness may be between 200 nm and 4000 nm, 300 nm and 3000 nm, 400 nmand 2000 nm, or between 1 μm and 10 μm, wherein the thickness may definethe smallest spatial expansion of the membrane) and a heating elementarranged (e.g. as a self-supporting bridge structure) on the membranebetween a first discontinuation area of the membrane and a seconddiscontinuation area of the membrane. The first discontinuation area ofthe membrane comprises at least one discontinuation of the membrane andthe second discontinuation area of the membrane comprises at least onediscontinuation of the membrane. In addition, the gas sensor maycomprise a first temperature sensor structure (e.g. “thermopilestructure”, temperature-variable resistors or thermistors) arranged atleast partially on the membrane on a side of the first discontinuationarea of the membrane opposite to the heating element (e.g. such that thefirst discontinuation area is arranged between the first temperaturesensor structure and the heating element and decreases thermalconduction in the membrane material from the heating element to thefirst temperature sensor structure, for example). In addition, the gassensor may comprise a second temperature sensor structure (e.g.“thermopile structure”, temperature-variable resistors or thermistors)arranged at least partially on the membrane on a side of the seconddiscontinuation area of the membrane opposite to the heating element(e.g. such that the second discontinuation area is arranged between thesecond temperature sensor structure and the heating element anddecreases thermal conduction in the membrane material from the heatingelement to the second temperature sensor structure, for example).

This embodiment is based on the finding that a gas (e.g. a gas to byanalyzed) that may conduct heat from the heating element to the firstthermal element structure and to the second thermal element structure ina gas-specific manner may be arranged in the at least onediscontinuation of the first discontinuation area and in the at leastone discontinuation of the second discontinuation area, respectively.Based on the runtimes of a heat transport by the gas from the heatingelement to the first temperature sensor structure and/or the secondtemperature sensor structure, properties of the gas may be detected bythe gas sensor. For example, a composition of the gas, a pressure of thegas, a speed of the gas, or a thermal conductivity of the gas may bedetected by means of the gas sensor.

In addition, the gas sensor may be realized to be very small. Due to thespecial arrangement of individual features of the gas sensor, severalproperties of a gas to be analyzed may be detected simultaneously, as aresult of which the gas sensor comprises a reduced installation space incomparison to gas sensor systems that have to integrate different gassensors in one system in order to detect several properties of a gas.The individual features of the gas sensor may be arranged such that thegas sensor comprises a very small size.

Thus, it is to be noted that, due to its special structure, the gassensor reduces installation space and system weight and may provide anexact flow measurement and a quick gas analysis.

According to an embodiment, the first temperature sensor structure is afirst thermal element structure having a hot end arranged on themembrane on a side of the first discontinuation area of the membraneopposite to the heating element, and the second temperature sensorstructure is a second thermal element structure having a hot endarranged on the membrane on a side of the second discontinuation area ofthe membrane opposite to the heating element. Thus, for example, thethermal element structures may comprise the hot end and a cold end on anopposite side of the thermal element structures. In this case, e.g., hot(of the term hot end) means that this side of the thermal elementstructures is arranged to face the heating element, and cold (of theterm cold end) means that this side of the thermal element structures isarranged to face away from the heating element, for example. Thus, forexample, the temperature sensor structure is arranged at least partially(with the hot end) on the membrane. However, it is also possible thatthe entire temperature sensor structure is arranged on the membrane withthe hot end and the cold end. According to an embodiment, the hot end ofthe temperature sensor structure is used for detecting a heat transferfrom the heating element to the respective temperature sensor structurevia the gas to be analyzed.

According to an embodiment, the first temperature sensor structurecomprises a same distance to the heating element as the secondtemperature sensor structure. For example, this makes it possible toreduce inaccuracies in the gas analysis, e.g., due to the fact that theresults of the two temperature sensor structures may be compared witheach other. Alternatively, a very accurate gas analysis may be achievedfrom a sum signal made up of a first signal detected by means of thefirst temperature sensor structure and a second signal detected by meansof the second temperature sensor structure.

According to an embodiment, for example, the temperature sensorstructure may be a “thermopile structure”, temperature-variableresistors or thermistors. Even if the following illustrates embodimentswith respect to a thermal element structure or “thermopile structure” asa temperature sensor structure, it is obvious to the person skilled inthe art that temperature-variable resistors or thermistors may also beused instead. In addition, the following embodiments, or modificationsof the embodiments, may be combined in any way and may in particularalso be combined with the previous embodiments.

A further embodiment concerns a gas sensor (e.g. in the form of a MEMScomponent) including a membrane (e.g. a thin-layer membrane whosethickness may be between 200 nm and 4000 nm, 300 nm and 3000 nm, 400 nmand 2000 nm, or between 1 μm and 10 μm, wherein the thickness may definethe smallest spatial expansion of the membrane) and a heating elementarranged (e.g. as a self-supporting bridge structure) on the membranebetween a first discontinuation area of the membrane and a seconddiscontinuation area of the membrane. The first discontinuation area ofthe membrane comprises at least one discontinuation of the membrane andthe second discontinuation area of the membrane comprises at least onediscontinuation of the membrane. For example, the at least onediscontinuation (e.g. of the first discontinuation area and/or of thesecond discontinuation area) comprises a longitudinal expansion inparallel to the heating element that is larger than a lateral expansionperpendicular to the heating element. In this case, for example, the atleast one discontinuation extends along an entire length of the heatingelement (e.g. wherein the length defines an expansion of a heatingelement side that is adjacent to the membrane, or to the first and/orthe second discontinuation area, and extends in parallel to the firstand/or the second discontinuation area). In addition, the gas sensor maycomprise a first thermal element structure (e.g. a “thermopilestructure”) having a hot end (e.g. the end that is arranged closer tothe heating element) arranged on the membrane on a side of the firstdiscontinuation area of the membrane opposite to the heating element(e.g. so that the first discontinuation area is arranged between the hotend of the first thermal element structure and the heating element anddecreases a thermal conduction in the membrane material from the heatingelement to the first thermal element structure, for example). Inaddition, the gas sensor may comprise a second thermal element structure(e.g. a “thermopile structure”) having a hot end (e.g. the end that isarranged closer to the heating element) arranged on the membrane on aside of the second discontinuation area of the membrane opposite to theheating element (e.g. so that the second discontinuation area isarranged between the hot end of the second thermal element structure andthe heating element and decreases thermal conduction in the membranematerial from the heating element to the second thermal elementstructure, for example).

The gas sensor is implemented to be advantageous in that the gas sensorcomprises the first discontinuation area and the second discontinuationarea, which makes it possible that the first thermal element structureand/or the second thermal element structure may comprise a differentdistance (e.g. space) to the heating element since the firstdiscontinuation area may comprise an expansion perpendicular to theheating element that differs from that of the second discontinuationarea, for example. In the heat transport from the heating element to thefirst thermal element structure and/or the second thermal elementstructure via the gas, unknown heat transfers may occur from the heatingelement into the gas to be analyzed and from the gas into the firstthermal element structure and the second thermal element structure,respectively. For example, when measuring the gas sensor with twodifferent distances between the heating element and the thermal elementstructures, the heat transfers may be almost identical, e.g., as aresult of which a difference of the heat transport detected by the firstthermal element structure and the heat transport detected by the secondthermal element structure essentially depends on the heat transport viathe gas in the first discontinuation area and the second discontinuationarea, respectively. Thus, possible inaccuracies of the gas sensor may beavoided and properties of the gas may be detected with high sensitivityusing the gas sensor. In this case, the first thermal element structureand/or the second thermal element structure may be used as a sensor ofthe gas sensor.

According to an embodiment, the membrane is spread out by a frame madeof a carrier material (e.g. the membrane may be carried by a frame madeof a carrier material or a substrate material) implemented such that acoefficient of temperature expansion of the membrane deviates from acoefficient of temperature expansion of the carrier material holding themembrane. Through this configuration, great forces may act onto theelements of the gas sensor. The inventive design of the gas sensorhaving discontinuations in the membrane makes it possible to usematerials with different coefficients of temperature expansion. Forexample, despite using materials with different coefficients oftemperature expansion, it is possible to minimize impairments or damagesof elements of the gas sensor and to therefore ensure a simple andcost-efficient fabrication of a gas sensor. Thus, for example, as iscommonly the case, materials with the same or very similar coefficientsof temperature expansion do not have to be used.

According to an embodiment, cold ends of the first thermal elementstructure and cold ends of the second thermal element structure may bearranged on the carrier material. For example, they are located wherethe membrane is carried by the carrier material. Thus, for example, thefirst thermal element structure or the second thermal element structuremay be implemented to be meander-shaped, as a result of which the hotends are arranged on the membrane to face the heating element, and thecold ends are arranged on the carrier material to face away from theheating element. Due to this arrangement, there may be a temperaturedifference between the hot ends and the cold ends of the first thermalelement structure and/or the second thermal element structure, as aresult of which there may be a charge transfer within the first thermalelement structure and/or the second thermal element structure. Thus, forexample, a voltage may be present at the first thermal element structureand/or the second thermal element structure if the thermal elementstructure is exposed to a temperature difference. This voltage dependson the temperature difference. Thus, it is possible to very preciselydetect heat conducted from the heating element to the thermal elementstructure by a gas to be analyzed.

According to an embodiment, the first discontinuation area of themembrane may comprise a continuous discontinuation whose longitudinalexpansion is large enough to fully cover the area between the firstthermal element structure and the heating element, and the seconddiscontinuation area of the membrane may also comprise a continuousdiscontinuation whose longitudinal expansion is large enough to fullycover the area between the second thermal element structure and theheating element. For example, a longitudinal expansion may be understoodto be an expansion in parallel to the heating element, or in parallel tothe first thermal element structure and/or the second thermal elementstructure. Thus, for example, the continuous discontinuation of thefirst discontinuation area and/or the second discontinuation area maycomprise a longitudinal expansion along the full length of the heatingelement, or a longitudinal expansion at least along the full length ofthe second thermal element structure. This makes it possible that almostthe entire, or even the entire, heat transport from the heating elementto the first thermal element structure or the second thermal elementstructure takes place via the gas to be analyzed, that a parasitic heatconduction via the membrane may therefore be minimized or eliminated.Thus, for example, the gas sensor is implemented to perform a veryprecise flow measurement and a quick and precise gas analysis.

According to an embodiment, a lateral expansion (e.g. in a directionperpendicular to a direction of a maximum expansion of the heatingelement, or in a direction from the heating element to the respectivethermal element structures) of the discontinuation (e.g. the continuousdiscontinuation or the at least one discontinuation) of the firstdiscontinuation area may differ from a lateral expansion of thediscontinuation (e.g. the continuous discontinuation or the at least onediscontinuation) of the second discontinuation area. Thus, for example,the heat has to cover a different path length from the heating elementto the first thermal element structure through the medium arranged inthe discontinuation of the first discontinuation area then from theheating element to the second thermal element structure through themedium arranged in the discontinuation of the second discontinuationarea. Due to the different lateral expansion, it may be possible thatunknown heat transfers (e.g. from the heating element to the gas, orfrom the gas to the first thermal element structure or to the secondthermal element structure) play almost no role or play no role at all inthe detection of properties of a gas by the gas sensor, which is whyproperties of the medium (e.g. gas, fluid, liquid) may be determinedvery precisely. In addition, the gas sensor may be adapted to individualrequirements. Thus, for example, a small lateral expansion results in agreat stability of the gas sensor, and a large lateral expansion, forexample, results in a high gas-dependent sensitivity. Thus, a mechanicalstability of the gas sensor in the long term operation or itssensitivity for the detection of properties of a gas may be improvedwith different lateral expansions, respectively.

According to an embodiment, the first thermal element structure maycomprise a different distance to the heating element than the secondthermal element structure. Thus, for example, the heat has to cover adifferent path length from the heating element to the first thermalelement structure than from the heating element to the second thermalelement structure. Thus, it may be possible that unknown heat transfers(e.g. from the heating element to the gas, or from the gas to the firstthermal element structure or to the second thermal element structure)play almost no role or play no role at all in the detection ofproperties of a gas by the gas sensor. A difference between a firstsensor signal detected by the first thermal element structure and asecond sensor signal detected by the second thermal element structuremay essentially depend on a heat transmission via a medium (e.g. gas,fluid, liquid) arranged in the discontinuations of the discontinuationareas. Thus, the gas sensor may very precisely detect properties of agas.

According to an embodiment, the first discontinuation area and thesecond discontinuation area may comprise several discontinuationsarranged such that a grid structure is created in the respectivediscontinuation area, wherein the discontinuations are arranged in rowsin parallel to the heating element, and the rows are arranged offset toeach other. Thus, for example, several discontinuations may be arrangedin the membrane, as a result of which the remaining membrane materialmay form the grid structure. The discontinuations are arranged in rowsin parallel to the heating element, e.g., which means that the rows arearranged in parallel to a direction of a maximum expansion of theheating element. In addition, the rows may be arranged offset to eachother, e.g., which means that the lateral ridges—formed by the membranematerial—of the grid structure (extending in the direction perpendicularto the heating element; from the heating element to the respectivethermal element structure) of successive rows are arranged offset toeach other. In other words, the several discontinuations may be arrangedsimilar to a stretching bond. For example, this causes a parasitic heatconduction in the membrane to pass through as long a path as possible.

The several discontinuations may be longitudinal discontinuations. Forexample, a direction of a maximum expansion of the longitudinalexpansion is perpendicular to a main direction of the heat conduction(e.g. from the heating element to the thermal element structures) with atolerance of ±20°. Thus, the grid structure makes it possible thatparasitic heat is not directly conducted from the heating element to thefirst thermal element structure and/or the second thermal elementstructure, but passes through winding paths through the grid structure.In contrast to the desired heat conduction via the gas to be analyzed inthe several discontinuations, this may make it possible that theparasitic heat conduction via the grid structure reaches the firstthermal element structure and/or the second thermal element structurewith a delay.

According to an embodiment, the first discontinuation area and thesecond discontinuation area may comprise several discontinuationsarranged such that a grid structure is created, wherein a path of a heatconduction through the membrane is longer than a direct path. Here, theheat conduction may be a parasitic heat conduction. For example, adirect path may be a straight path perpendicular to a maximum expansionof the heating element, from the heating element to the first thermalelement structure and/or to the second thermal element structure, or apath through the gas in the several discontinuations. For example, thepath of the parasitic heat conduction through the membrane should notextend in a straight line, but should form a winding path. For example,there should be no direct heat path through the membrane. This makes itpossible that parasitic heat reaches the first thermal element structureand/or the second thermal element structure through the membrane with adelay with respect to a heat transport via the gas to be analyzed in theseveral discontinuations.

According to an embodiment, the discontinuations in the firstdiscontinuation area and in the second discontinuation area may berectangular cutouts having rounded corners. In other words, the at leastone discontinuation in the first discontinuation area and the at leastone discontinuation in the second discontinuation area may form arectangular cutout having rounded corners. For example, this may be alongitudinal hole or an oval hole. This makes it possible that a gridstructure created by the at least one discontinuation in the firstdiscontinuation area and/or in the second discontinuation area providesa long path for a parasitic heat conduction through the membrane. Thus,the gas sensor is configured to detect properties of a gas with a highaccuracy.

According to an embodiment, the discontinuations in the firstdiscontinuation area and in the second discontinuation area may be atleast three times longer than they are wide. In other words, the atleast one discontinuation in the first discontinuation area and the atleast one discontinuation in the second discontinuation area may be atleast three times longer than they are wide. For example, the length isdefined as a direction in parallel to a maximum expansion of the heatingelement and the width is defined as a direction perpendicular to themaximum expansion of the heating element, or a direction from theheating element to the first thermal element structure and/or the secondthermal element structure. Thus, for example, the at least onediscontinuation of the first discontinuation area and/or the at leastone discontinuation of the second discontinuation area is configured toensure a grid structure having as long a parasitic heat conductionthrough the membrane as possible.

According to an embodiment, a distance between the discontinuations inthe first discontinuation area and a distance between thediscontinuations in the second discontinuation area may correspond tothe smallest realizable structural width resulting in a mechanicallydurable grid structure. For example, the distance between thediscontinuations in the first discontinuation area and the distancebetween the discontinuations in the second discontinuation area maydefine the width of ridges made of a membrane material, which may form agrid structure in the respective discontinuation area. For example, thewidth of the ridges defines an expansion of the ridges perpendicular tothe discontinuations adjacent to the respective ridge. Thus, forexample, the width defines the distance between two adjacentdiscontinuations in the first discontinuation area and/or in the seconddiscontinuation area. For example, the distance between thediscontinuations in the first discontinuation area and the distancebetween the discontinuations in the second discontinuation area may bein a range from 10 nm to 1 mm, 100 nm to 1 μm, or 1 μm to 100 μm.Starting from 100 μm, the distance is very probably larger than theboundary layer area of the gas on the ridge side face. For example, heatconduction predominately takes place within the boundary layer, thethermal capacity c_(p) of the gas only applies outside of the boundarylayer. The boundary layer width depends on the absolute temperature (andthe gas). A quotient of an effective discontinuation face with respectto an effective ridge length may be considered as the aspect ratio. Forexample, for the sake of simplicity, if the discontinuation is 3× longerthan it is wide, 3 squares may be assumed as an effectivediscontinuation face. Analogously to a plate capacitor withC=epsilon*A/d. As an effective ridge length, the ridge connections maybe divided into equal squares having the edge length of the ridge width,for example, wherein the ridges in the main direction of the temperaturegradient weigh more than the ridges parallel to the heater. Analogouslyto the electric equivalent circuit diagram, the ridges may constitute aresistance path R, wherein R should be high. For example, the aspectratio includes 1/R, i.e. the electric conductance. If this is consideredas a R-C network, it may be tuned to a certain excitation frequency andgas type. However, for example, they are low pass filters with differentruntimes (hole and ridge). Thus, makes it possible that the gas sensorcomprises a great mechanical stability and simultaneously reduces, oreliminates, a parasitic heat conduction via the distances of thediscontinuations (consisting of membrane material). The larger theaspect ratio between the discontinuation face and the ridge width, thegreater the gas type-dependent sensitivity of the gas sensor, forexample.

According to an embodiment, the heating element, the first thermalelement structure and/or the second thermal element structure may bepassivated with a protective layer. For example, this enables a highresistance of the gas sensor against free radicals, e.g., that may bepresent in a measuring gas, e.g., that may be arranged in the at leastone discontinuation of the first discontinuation area and/or in the atleast one discontinuation of the second discontinuation area. Forexample, the free radicals may etch the sensitive active first thermalelement structure, the sensitive active second thermal element structureand/or the heating element and therefore mechanically weaken orthermally change the same, which may be prevented, or reduced, by meansof the protective layer.

According to an embodiment, the hot end of the first thermal elementstructure may reach up to an edge of the first discontinuation area ofthe membrane, and the hot end of the second thermal element structuremay reach up to an edge of the second discontinuation area of themembrane. For example, the first end of the first thermal elementstructure and/or the second thermal element structure includes severalends comprising a same or slightly different distance to the heatingelement. For example, the distance between the hot end of the thermalelement structures and the respective discontinuation area should not belarger than 0.5 mm, 0.1 mm, 50 μm, or 1 μm. Thus, this makes it possibleto arrange the first thermal element structure and/or the second thermalelement structure very closely to at least one discontinuation of therespective discontinuation area. Through this, the first thermal elementstructure and/or the second thermal element structure may very quicklydetect a heat transport carried out via the gas to be analyzed (arrangedin the discontinuation). Thus, the gas sensor is configured to veryquickly detect properties of a gas.

An embodiment provides a method for operating a gas sensor, wherein thegas sensor may be a gas sensor according to one of the embodimentsdescribed herein. The method may comprise heating a heating element andconducting heat via a gas mixture, wherein more heat is conducted fromthe heating element to a thermal element structure via the gas mixturesurrounding the gas sensor than via a membrane. In this case, the gasmixture may be arranged in a discontinuation of the membrane of the gassensor. In addition, the method may comprise detecting a heat transferby the hot ends of a thermal element structure.

In other words, the method may comprise heating a heating element andconducting heat via a gas mixture, wherein more heat is conducted fromthe heating element to a first thermal element structure and/or to asecond thermal element structure via the gas mixture surrounding the gassensor than via a membrane. In addition, the method may comprisedetecting a heat transfer by the hot ends of the first thermal elementstructure and/or the second thermal element structure.

According to an embodiment, the method includes determining a gasconcentration and/or a gas composition and/or a gas flow on the basis ofthe detection of the heat transfer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1a shows a schematic illustration of a gas sensor according to anembodiment of the present invention;

FIG. 1b shows a schematic illustration of an evaluation arrangement fora thermal gas sensor according to an embodiment of the presentinvention;

FIG. 1c shows a schematic illustration of an evaluation arrangement fora thermal gas sensor with a control of a heating power, according to anembodiment of the present invention;

FIG. 1d shows a schematic illustration of an evaluation arrangement fora thermal gas sensor with sampling a sensor signal at three points intime, according to an embodiment of the present invention;

FIG. 2a shows a schematic illustration of a gas sensor in the lightmicroscope, according to an embodiment of the present invention;

FIG. 2b shows a schematic illustration of a gas sensor in the scanningelectron microscope, according to an embodiment of the presentinvention;

FIG. 3 shows a schematic illustration of a section of a scanningelectron microscope image of micro bridge for a gas sensor according toan embodiment of the present invention;

FIG. 4 shows a schematic illustration of a gas sensor with a firstdiscontinuation having an expansion perpendicular to a heater thatdiffers from an expansion perpendicular to a heater of a seconddiscontinuation, according to an embodiment of the present invention;

FIG. 5 shows a schematic illustration of a gas sensor with a firstdiscontinuation area and a second discontinuation area each having aplurality of discontinuations, according to an embodiment of the presentinvention;

FIG. 6a shows a schematic illustration of a gas sensor with an equalnumber of discontinuations in the first discontinuation area and in thesecond discontinuation area, according to an embodiment of the presentinvention;

FIG. 6b shows a schematic illustration of a gas sensor with a multitudeof discontinuations in a first discontinuation area and a singlediscontinuation in a second discontinuation area, according to anembodiment of the present invention;

FIG. 6c shows a schematic illustration of a gas sensor, wherein amultitude of discontinuations in a first discontinuation area comprise adifferent expansion perpendicular to a heater than a multitude ofdiscontinuations in a second discontinuation area, according to anembodiment of the present invention;

FIG. 7 shows a schematic illustration of a principle of a gas sensoraccording to an embodiment of the present invention;

FIG. 8 shows a schematic illustration of a heat transport at a gassensor according to an embodiment of the present invention;

FIG. 9 shows a diagram of a heater signal, a first sensor signal, and asecond sensor signal of a gas sensor according to an embodiment of thepresent invention;

FIG. 10 shows a schematic illustration of driving a heater for a gassensor according to an embodiment of the present invention;

FIG. 11 shows a schematic illustration of a circuit for evaluating asensor signal of a gas sensor according to an embodiment of the presentinvention;

FIG. 12 shows a schematic illustration of a control of a gas sensoraccording to an embodiment of the present invention;

FIG. 13a shows a block diagram of a method for analyzing a sensor signalof a gas sensor according to an embodiment of the present invention;

FIG. 13b shows a block diagram of a method for evaluating a sensorsignal of a gas sensor with tracking sampling times, according to anembodiment of the present invention;

FIG. 14 shows a diagram of a phase shift between a heater signal and twosensor signals of a gas sensor according to an embodiment of the presentinvention;

FIG. 15 shows a diagram of amplitudes of at least one sensor signal of agas sensor according to an embodiment of the present invention;

FIG. 16 shows a diagram of phase shifts between a first sensor signaland a second sensor signal of a gas sensor as a function of a pressure,according to an embodiment of the present invention;

FIG. 17a shows a diagram of a phase shift of a sensor signal of a gassensor as a function of a frequency, according to an embodiment of thepresent invention;

FIG. 17b shows a diagram of an amplitude of sensor signal of a gassensor as a function of a frequency, according to an embodiment of thepresent invention;

FIG. 18 shows a diagram of phase shifts of a first sensor signal, asecond sensor signal, and a heater signal of a gas sensor as a functionof a nitrogen concentration, according to an embodiment of the presentinvention;

FIG. 19 shows a diagram of an amplitude of a first sensor signal and asecond sensor signal of a gas sensor as a function of a nitrogenconcentration, according to an embodiment of the present invention;

FIG. 20 shows a diagram of a combination signal of a gas sensor fordifferent gas mixtures, according to an embodiment of the presentinvention;

FIG. 21 shows a diagram of a combination signal of a gas sensor as afunction of a CO₂ concentration, according to an embodiment of thepresent invention;

FIG. 22 shows a diagram of a combination signal of a gas sensor as afunction of a pressure, according to an embodiment of the presentinvention;

FIG. 23 shows a diagram of a relationship between a gas pressure and agas temperature for a gas sensor according to an embodiment of thepresent invention;

FIG. 24 shows a block diagram of a method for generating a combinationsignal of a gas sensor according to an embodiment of the presentinvention; and

FIG. 25 shows a diagram of a thermal diffusivity as a function of acombination signal of a sensor according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Before embodiments of the present invention are subsequently describedin more detail based on the drawings, it is to be noted that elements,objects, and/or structures that are identical, functionally identical orhave the same effect are provided in the different drawings with thesame or similar reference numerals so that the description of theseelements illustrated in different embodiments may be interchangeable orapplicable to each other.

FIG. 1 shows a schematic illustration of a gas sensor 100 according toan embodiment of the present invention. The gas sensor 100 may comprisea membrane 110 (e.g. a thin-layer membrane), a heating element 120, afirst thermal element structure 130, and a second thermal elementstructure 140. Optionally, the gas sensor may only comprise the firstthermal element structure 130 or the second thermal element structure140. The membrane 110 may be spread out by a frame 150 and may comprisea first discontinuation area 160 and a second discontinuation area 170.The first discontinuation area 160 of the membrane 110 may comprise atleast one discontinuation 162, and the second discontinuation area 170of the membrane 110 may also comprise at least one discontinuation 172.For example, the heating element 120 may be arranged as aself-supporting bridge structure on the membrane 110 between the firstdiscontinuation area 160 and the second discontinuation area 170 of themembrane 110. The first thermal element structure 130 may comprise a hotend 132 and a cold end 134. The hot end 132 of the first thermal elementstructure 130 may be arranged on the membrane 110 on a side of the firstdiscontinuation area 160 opposite to the heating element 120. The secondthermal element structure 140 may also comprise a hot end 142 and a coldend 144. The hot end 142 may be arranged on the membrane 110 on a sideof the second discontinuation area 170 opposite to the heating element120.

The membrane 110 may be a thin-layer membrane with a thickness between200 nm and 4000 nm, 300 nm and 3000 nm, 400 nm and 2000 nm, or 1 μm and10 μm. According to an embodiment, the thickness of the overall membraneis approximately 2 μm (e.g., it consists of several membrane layers,sensor layers, and passivation layers). For example, the membrane layermay comprise Si oxide and/or Si nitride. For example, an expansion ofthe membrane 110 into the sheet plane, i.e. perpendicular to a surfaceof the membrane 110 on which the heating element 120, the first thermalelement structure 130, and the second thermal element structure 140 arearranged, may be defined as the thickness. The membrane 110 may comprisea conducting material, an insulating material, or a semiconductormaterial, wherein the material may comprise a very low thermalconductivity of below 5 W/(m*K), below 100 mW/(m*K), or below 50mW/(m*K), for example. For example, a semiconductor with adapted basicdoping may serve as a cost-efficient substrate for manufacturing themembrane 110 in a simple five mask MEMS process.

According to an embodiment, the heating element 120 (in the following,the heating element 120 may also be referred to as a heater) may form aself-supporting bridge structure and/or may include a wire. According toan embodiment, the heating element 120 may be spread out from one sideof the frame 150 to an opposite side of the frame 150. For example, avoltage may be applied to the heating element 120, as a result of whichthe heating element 120 may transmit a heating power to a gas to beanalyzed, e.g., that is located in the first discontinuation area 162and/or in the second discontinuation area 172. For example, the voltageapplied to the heating element 120 may be a periodic voltage signal suchas a sinusoidal signal or a periodic square-wave signal. Thus, forexample, the heating element 120 may provide a periodic heater signal(e.g. the heating power). For example, the heater signal may betransmitted to the first thermal element structure 130 and/or the secondthermal element structure 140 via the membrane 110 and/or via a gaslocated in the first discontinuation 162 or the second discontinuation172, for example.

For example, the first thermal element structure 130 and/or the secondthermal element structure 140 are configured to be meander-shaped, whichmay correspond to thermal elements connected in series and forming athermopile, for example. Thus, the first thermal element structure 130and/or the second thermal element structure 140 may serve as a detector,wherein the first thermal element structure 130 and/or the secondthermal element structure 140 may detect the heater signal, for example.

According to an embodiment, the first thermal element structure 130and/or the second thermal element structure 140 may be arranged entirelyon the membrane 110, or may be arranged at least partially on themembrane 110 and at least partially on the frame 150. Thus, for example,a temperature of the frame 150 may be used as a comparison temperature(e.g. the cold ends 134 of the first thermal element structure 130 maybe arranged here and/or the cold ends 144 of the second thermal elementstructure 140 may be arranged here), and the part of the thermal elementstructure arranged on the membrane 110 (e.g. the hot ends 132, 142) maydetect a measurement temperature (e.g. the heater signal). For example,the hot ends 132, 142 and the cold ends 134, 144 are connected viaconductor. Thus, for example, a conductor including a first material mayconnect a first cold end to a first hot end, and a second conductorincluding a second material may connect the first hot end to a secondcold end. This connection of a first conductor and a second conductormay constitute a thermal element, e.g., which may be connected in seriesto form a thermopile and which therefore may constitute the firstthermal element structure 130 or the second thermal element structure140, for example. Thus, for example, a temperature difference (e.g.between the comparison temperature and the measurement temperature) mayoccur along these conductors, as a result of which, e.g., an electricvoltage may be induced at the ends (e.g. the hot ends and/or the coldends) of the metal conductors. Thus, for example, the first thermalelement structure 130 and/or the second thermal element structure 140may be configured to convert heat into electrical energy. According toan embodiment, the first thermal element structure 130 and/or the secondthermal element structure 140 may be a wire or a self-supporting bridgestructure.

According to an embodiment, the membrane 110 may be spread out by theframe 150 made of a carrier material that is implemented such that thecoefficient of temperature expansion and/or a thermal conductivity of amembrane material deviates from the coefficient of thermal expansionand/or the thermal conductivity of the carrier material. The frame 150may comprise a carrier material or a substrate material with which themembrane 110 may be carried, for example, Thus, for example, acomparison temperature may be set at the frame 150. According to anembodiment, the frame 150 and the membrane 110 may also comprise thesame coefficient of thermal expansion.

According to an embodiment, the membrane 110 may comprise a lowerthermal conductivity than the frame 150. In this case, for example, themembrane 110 should in particular comprise a very low thermalconductivity so that the heater signal is transmitted from the heatingelement 120 to the first thermal element structure 130 and/or to thesecond thermal element structure 140 mainly via the gas to be analyzed(e.g. arranged in the first discontinuation 162 and/or in the seconddiscontinuation 172) instead of via the membrane 110. Thus, for example,a heat transport via the membrane 110 may be suppressed, reduced, orslowed down.

Thus, the membrane 110 may be configured to suppress parasitic thermalconduction from the heating element 120 to the first thermal elementstructure 130 and/or to the second thermal element structure 140. Thus,for example, the thermal conductivity of the membrane 110 may beselected such that little to no heat is conducted from the heatingelement 120 to the first thermal element structure 130 and/or the secondthermal element structure 140 via the membrane 110 and such that amajority of the heat, or the entire heat, is conducted via the gas to beanalyzed.

On the other hand, the thermal conductivity of the carrier material ofthe frame 150 holding the membrane 110 may be very high. Thus, forexample, silicon having a thermal conductivity of 150 W/(m*K) may beused as the carrier material. Thus, the carrier material may be used asa heat sink. Thus, for example, the first thermal element structure 130and/or the second thermal element structure 140 is arranged partially,e.g. with the hot ends 132, 142, on the membrane and at least partially,e.g. with the cold ends 134, 144, on the carrier material, as a resultof which a temperature difference may occur within the first thermalelement structure 130 and/or the second thermal element structure 140,with the help of which the heat transport from the heating element 120to the respective thermal element structure 130, 140 may be detected.

Thus, according to an embodiment, the cold ends of the first thermalelement structure 130 and the cold ends of the second thermal elementstructure 140 may be arranged on the carrier material of the frame 150.For example, they are located where the membrane 110 is carried by thecarrier material.

According to an embodiment, the first discontinuation area 160 of themembrane 110 may comprise a continuous discontinuation 162 whoselongitudinal expansion 164 is large enough to fully cover the areabetween the first thermal element structure 130 and the heating element120. The second discontinuation area 170 of the membrane 110 maycomprise a continuous discontinuation 172 whose longitudinal expansion174 is large enough to fully cover the area between the second thermalelement structure 140 and the heating element 120. Thus, for example,the longitudinal expansion 164, 174 is as large as the entire length ofthe heating element 120 and/or at least as large as the entire length ofthe first thermal element structure 130 and/or the second thermalelement structure 140. Thus, this makes it possible to transmit aslittle heat as possible from the heating element 120 to the firstthermal element structure 130 or the second thermal element structure140 via the membrane 110, but a majority is transmitted via a gas in thefirst discontinuation 162 in the first discontinuation area 160 and/orin the second discontinuation 172 in the second discontinuation area170.

According to an embodiment, the lateral expansion 166 of the at leastone discontinuation 162 of the first discontinuation area 160 may differfrom the lateral expansion 176 of the at least one discontinuation 172of the second discontinuation area 170. For example, the lateralexpansion 166, 176 of the first discontinuation 162 and the seconddiscontinuation 172, respectively, may be directed in a directionperpendicular to the a direction of a maximum expansion of the heatingelement 120, or in a direction from the heating element 120 to therespective thermal element structure (e.g. the first thermal elementstructure 130 and/or the second thermal element structure 140). Forexample, according to FIG. 1a , the first discontinuation 162 and thesecond discontinuation 172 comprise the same lateral expansion 166, 176.

According to an embodiment, the first discontinuation 162 may comprise alongitudinal expansion 164 and a lateral expansion 166 so that the firstdiscontinuation 162 corresponds to the expansions of the firstdiscontinuation area 160. Similarly, for example, the seconddiscontinuation 172 may comprise a longitudinal expansion 174 and alateral expansion 176 so that the second discontinuation 172 correspondsto the expansions of the second discontinuation area 170. Thus, forexample, the entire first discontinuation area 160 may constitute thefirst discontinuation 162, and the entire discontinuation area 170 mayconstitute the discontinuation 172.

Optionally, on the side of the cold ends 134, 144 of the first thermalelement structure 130 and/or the second thermal element structure 140,the membrane 110 may comprise a third and/or a fourth discontinuationarea. Thus, for example, the first thermal element structure 130 may bearranged in the form of a wire or as a self-supporting bridge structurebetween the first discontinuation area 160 and a third discontinuationarea, and/or the second thermal element structure 140 may be arranged asa wire or as a self-supporting bridge structure between the seconddiscontinuation area 170 and the fourth discontinuation area, forexample. Thus, for example, the first thermal element structure 130and/or the second thermal element structure 140 may be surrounded fromtwo sides by the gas to be analyzed.

According to an embodiment, the first thermal element structure 130 maycomprise a different distance to the heating element 120 than the secondthermal element structure 140. For example, in FIG. 1a , the firstthermal element structure 130 comprises the same distance to the heatingelement 120 as the second thermal element structure 140. Whentransmitting the heater signal from the heating element 120 to the firstthermal element structure 130 via the first discontinuation 162 and/orfrom the heating element 120 to the second thermal element structure 140via the second discontinuation 172, unknown heat transfers may occurfrom the heating element into the gas to be analyzed that is arranged inthe first discontinuation 162 and/or in the second discontinuation 172,and from the gas to the first thermal element structure 130 and/or thesecond thermal element structure 140. For example, the heater signalfrom the heating element 120 that is detected by the first thermalelement structure 130 may be referred to as first sensor signal, and theheater signal from the heating element 120 that is detected by thesecond thermal element structure 140 may be referred as second sensorsignal, for example.

For example, the first sensor signal and/or the second sensor signal maycomprise the two unknown heat transitions (e.g. heating element->gas,gas->thermal element structure) and a heat transfer via the gas to beanalyzed. If the first thermal element structure 130 is spaced apartfrom the heating element 120 differently than the second thermal elementstructure 140, for example, the gas sensor may create a differencesignal from the first sensor signal and the second sensor signal, e.g.,in which the unknown heat transitions (the first sensor signal and thesecond sensor signal may comprise the same heat transitions) may besubtracted out, and the difference signal therefore only, or to a largepart, comprises the heat transfer from the heating element 120 to therespective thermal element structure 130, 140 via the gas to beanalyzed, but does not, or only to a very small part, comprise theunknown heat transfers.

According to an embodiment, the first discontinuation area 160 and thesecond discontinuation area 170 may comprise several discontinuations(e.g. the discontinuation 162 and the discontinuation 162 ₁, or thediscontinuation 172 and the discontinuation 172 ₁) that may be arrangedsuch that (e.g. by the remaining membrane material 110) a grid structureis created (e.g. in the first discontinuation area 160 or the seconddiscontinuation area 170) in which the discontinuations are arranged inrows in parallel to the heating element 120, and the rows are arrangedto be offset to each other. In this case, the discontinuations in adiscontinuation area 160, 170 may differ from each other with respect tothe longitudinal expansion 164, 174 and the lateral expansion 166, 176.For example, according to FIG. 1a , the discontinuation 162 ₁ of thefirst discontinuation area 160 comprises a smaller longitudinalexpansion than the longitudinal expansion 164 of the discontinuation162. Similarly, the discontinuation 172 ₁ of the second discontinuationarea 170 may comprise a smaller longitudinal expansion than thelongitudinal expansion 174 of the discontinuation 172.

According to an embodiment, the first discontinuation area 160 and thesecond discontinuation area 170 may comprise several discontinuationsthat may be arranged such that a grid structure is created in which apath of a heat conduction by the membrane 110 is longer than a directpath 122 a, 122 b. For example, the direct path 122 a, 122 b may be astraight path perpendicular to the heating element 120, from the heatingelement 120 to the thermal element structure 130, 140. In this case, thedirect path 122 a, 122 b may pass through the discontinuations 162 and162 ₁ and the discontinuations 172 and 172 ₁, respectively, as a resultof which a heat conduction by the gas to be analyzed may be sensed bythe first thermal element structure 130 and/or the second thermalelement structure 140. If the direct path 122 a, 122 b were to takeplace only via the membrane 110 and not via the gas to be analyzed, thegas sensor 100 could not ensure a meaningful analysis of the gas.

According to an embodiment, the at least one discontinuation 162, 172may form rectangular cutouts with optionally rounded corners in thefirst discontinuation area 160 and the second discontinuation area 170.In this case, for example, it is a longitudinal hole. For example, itmay also be an oval hole. Even though the discontinuation 162 of thefirst discontinuation area 160 and the discontinuation 172 of the seconddiscontinuation area 170 are illustrated as rectangular discontinuations(holes) in FIG. 1a , the discontinuations may comprise any shapes (suchas triangular, circular, square, polygon-shaped, etc.). The shaping ofthe discontinuations 162, 172 may be adapted such that a heat path fromthe heating element to the first thermal element structure 130 and/or tothe second thermal element structure 140 via the membrane 110 is as longas possible, and a path via the gas to be analyzed constitutes a verylong route. Thus, this makes it possible to transport as much heat aspossible via the gas to be analyzed and not via the membrane 110, as aresult of which the gas sensor 100 may very precisely analyze the gas.

According to an embodiment, the at least one discontinuation 162, 172may be at least three times longer than it is wide. Thus, for example,the longitudinal expansion 164 of the discontinuation 162 may be threetimes longer than the lateral expansion 166, or the longitudinalexpansion 174 of the discontinuation 172 may be three times longer thanthe lateral expansion 176. Thus, for example, the length constitutes thelongitudinal expansion 164, 174, and the width constitutes the lateralexpansion 166, 176, for example. For example, the length may be definedas a direction in parallel to the heating element 120 (or to a directionof maximum expansion of the heating element 120), and the width may bedefined as a direction perpendicular to the heating element 120 (or to adirection of maximum expansion of the heating element 120).

According to an embodiment, a distance 168 between severaldiscontinuations 162, 162 ₁ in the first discontinuation area 160, and adistance 178 between several discontinuations 172, 172 ₁ in the seconddiscontinuation area 170 may correspond to the smallest realizablestructural width that results in a mechanically durable grid structure.The distance 168, 178 may define a width of ridges between twodiscontinuations, and consisting of membrane material of the membrane110. The smaller the distance 168, 178, the less heat is transferred viathe membrane 110 from the heating element 120 to the first thermalelement structure 130 and/or the second thermal element structure 140,and the more heat is transferred via the gas to be analyzed.

According to an embodiment, the first thermal element structure 130 andthe second thermal element structure 140 may be passivated with aprotective layer. The protective layer may protect the first thermalelement structure 130 and the second thermal element structure 140against damages by the gas to be analyzed, and may therefore avoidpossible inaccuracies of the gas sensor in the gas analysis due todamages of the first thermal element structure 130 and/or the secondthermal element structure 140.

According to an embodiment, the hot end 132 of the first thermal elementstructure may reach up to an edge of the first discontinuation area 160of the membrane 110, and the hot end 142 of the second thermal elementstructure 140 may reach up to an edge of the second discontinuation area170 of the membrane 110. For example, the distance between the hot end132 and the first discontinuation area 160, or the distance between thehot end 142 and the second discontinuation area 170, should not belarger than 0.5 mm, 100 nm, or 10 μm. For example, if thediscontinuation 162 or the discontinuation 172 reaches up to this edge,the membrane 110 has only a very small distance between the respectivehot ends and the respective discontinuation. This makes it possible thatthe membrane material of the membrane 110 does not or only slightlyimpair a detection of the heater signal by the first thermal elementstructure 130 or the second thermal element structure 140, as a resultof which the gas sensor 100 may very precisely analyze the gas.

FIG. 1b shows a schematic illustration of an evaluation arrangement 200for a thermal gas sensor 100 with at least one heater 120 and twodetectors (a first detector 130 and a second detector 140) arranged indifferent distances 180 ₁ and 180 ₂ to the heater 120. The firstdetector 130 may be spaced apart from the heater 120 with a distance 180₁, and the second detector 140 may be spaced apart from the heater 120with a distance 180 ₂. The evaluation arrangement 200 may be implementedto obtain information 210 about an amplitude of a detector signal of afirst detector 130, information 220 about an amplitude of a detectorsignal of a second detector 140, information 210 about a first phasedifference between a heater signal and the detector signal of the firstdetector 130, and information 220 about a second phase differencebetween the heater signal and the detector signal of the second detector140.

According to an embodiment, the information 210 may include theamplitude of the detector signal of the first detector 130 as well asthe first phase difference between the heater signal and the detectorsignal of the first detector 130, and the information 220 may includethe amplitude of the detector signal of the second detector 140 as wellas the second phase difference between the heater signal and thedetector signal of the second detector 140. However, it is also possiblethat the amplitude of the detector signal of the respective detector(the first detector 130 and/or the second detector 140) is transmittedseparately from the first phase difference and the second phasedifference, respectively, from the thermal gas sensor to the evaluationarrangement. According to an embodiment, it is also possible that theinformation 210 and the information 220 are not transmitted via separatelines to the evaluation arrangement 200, but via a mutual line orwireless, for example.

According to an embodiment, the evaluation arrangement 200 may beimplemented to form a combination signal 230 as an intermediate quantitydependent on the information 210, 220 about the amplitudes of thedetector signals and dependent on the information 210, 220 about thefirst phase difference and the second phase difference. The combinationsignal 230 may combine amplitude information and phase information ofthe detector signal of the first detector 130 and of the detector signalof the second detector 140. The evaluation arrangement 200 may beimplemented to determine information 240 about a gas concentration or athermal diffusivity of a fluid, such as a gas or as a gas mixture, basedon the combination signal 230. For example, the evaluation arrangement200 may perform this determination without separately reconsidering theindividual information 210, 220 incorporated into the combination signal230 in the further process of the calculations.

For example, the amplitude of the detector signal may be directlyprovided as information 210, 220 by the respective detector 130, 140.The information 210, 220 about the first phase difference and the secondphase difference between the heater signal 122 and the detector signalof the respective detector 130, 140 may be determined by the thermal gassensor 100 and be transmitted to the evaluation arrangement 200, forexample.

Alternatively, the detector signal of the first detector 130 and thedetector signal of the second detector 140 may be transmitted to theevaluation arrangement 200 via the information 210 and the information220, respectively, and the heater signal 122 may be additionallytransmitted directly to the evaluation arrangement 200. In this case,the evaluation arrangement may be configured to determine the respectiveamplitude from the detector signal of the first detector 130 and fromthe detector signal of the second detector 140, and to determine thefirst phase difference and the second phase difference in order to formthe combination signal 230 dependent on the information determined insuch a way.

Due to the fact that the evaluation arrangement 200 forms thecombination signal 230, the evaluation arrangement 200 may easily andmuch more quickly correct possible errors of the thermal gas sensor 100to obtain the information 240 about the gas concentration and a thermaldiffusivity, as would be the case if the evaluation arrangement 200would separately correct the information 210 about the amplitude of thedetector signal of the first detector 130 and the first phase differenceas well as the information 220 about the amplitude of the detectorsignal of the second detector 140 and the second phase difference. Thus,the combination signal 230 may facilitate determining the information240 about the gas concentration and the thermal diffusivity of the gasto be analyzed, and makes it possible to suppress or reduce errorsgenerated by the thermal gas sensor 100.

According to an embodiment, the evaluation arrangement 200 may beconfigured to obtain information about a heater amplitude, such asinformation about a heating power, from the heater signal 122 and toform a linear combination of the information about the heater amplitude,the information 210 and the information 220 in order to obtain thecombination signal 230.

Alternatively, the evaluation arrangement 200 may not only obtain theinformation about the heater amplitude from the heater signal 122 butmay also, as described above, calculate information about the firstphase difference and the second phase difference, e.g., if theinformation 210 includes the detector signal of the first detector 130and the information 220 includes the detector signal of the seconddetector 140.

Thus, it is not only the phase of the heater signal that is incorporatedinto the combination signal 230 in the form of the first phasedifference and the second phase difference, but also the heateramplitude, which makes it possible that the evaluation arrangement 200may determine the information 240 about the gas concentration and thethermal diffusivity of the gas to be analyzed dependent on the firstdistance 180 ₁ and the second distance 180 ₂ of the two detectors fromthe heater 120. Thus, for example, the detector signal of the firstdetector 130 comprises a larger amplitude than the detector signal ofthe second detector 140 since the distance 180 ₂ of the second detector140 to the heater 120 is larger than the distance 180 ₁ of the firstdetector 130 to the heater 120. With increasing distance to the heater120, the heater amplitude detected by the respective detector 130, 140may decrease. Due to the additional information about the heateramplitude, the evaluation arrangement 200 may therefore determine theinformation 240 about the gas concentration and the thermal diffusivityeven more precisely since the heater amplitude of the heater signal 122may be considered as a reference, and the combination signal 230 maytherefore comprise a relative amplitude signal. For example, a relativeamplitude signal is less error-prone than an absolute amplitude signal.

According to an embodiment, the evaluation arrangement 200 may beimplemented to obtain the combination signal sigX 230 according tosigX=sigUss*Ka+sigPhi*Kp. The term sigUss may be amplitude informationor an amplitude signal that may depend on the information 210 about theamplitude of the detector signal of the first detector 130 and on theinformation 220 about the amplitude of the detector signal of the seconddetector 140. For example, sigUss may be linear combination of theinformation 210 about the amplitude of the detector signal of the firstdetector 130 and the information 220 about the amplitude of the detectorsignal of the second detector 140. sigPhi may be phase information or anadded phase signal that may depend on the information 210 about a firstphase difference and on the information 220 about the second phasedifference. Thus, for example, sigPhi may be an addition of theinformation 210 about the first phase difference and the information 220about the second phase difference. Ka and Kp may be constants. Thecombination signal 230 determined in such a way may include amplitudeinformation sigUss and phase information sigPhi, as a result of whichfour pieces of information (e.g. the information 210 about the amplitudeof the detector signal of the first detector 130, the information 220about the amplitude of the detector signal of the second detector 140,the information 210 about a first phase difference between the heatersignal and the detector signal of the first detector 130, and theinformation 220 about the second phase difference between the heatersignal and the detector signal of the second detector 140) may becombined in the combination signal 230, as a result of which theevaluation arrangement 200 may use less power for processing theinformation 210, 220. Thus, the evaluation arrangement 200 may beconfigured to determine information 240 about the gas concentration andthermal diffusivity very efficiently, quickly and precisely.

According to an embodiment, the evaluation arrangement 200 may beconfigured to obtain the amplitude information sigUss according tosigUss=2*Hz.Uss−(D1.Uss+D2.Uss). Hz.Uss may be information about theheater amplitude that may be obtained from the heater signal 122. D1.Ussmay be information 210 about the amplitude of the detector signal of thefirst detector 130, and D2.Uss may be information 220 about theamplitude of the detector signal of the second detector 140. Thus, theamplitude information sigUss may constitute a relative amplitude signalsince the information 210 about the amplitude of the detector signal ofthe first detector 130, the information 220 about the amplitude of thedetector signal of the second detector 140, and the heater amplitudeHz.Uss are calculated with each other so that the information 210 aboutthe amplitude of the detector signal of the first detector 130 and theinformation 220 about the amplitude of the detector signal of the seconddetector 140 may be considered relative to the heater amplitude. Due tothe relative consideration of the amplitudes, possible errors ofabsolute amplitude values may be avoided, as a result of which theevaluation arrangement 200 may very precisely determine the information240 about the gas concentration and thermal diffusivity.

According to an embodiment, the evaluation arrangement 200 may beimplemented to calculate a polynomial, e.g. of the first degree, of thecombination signal 230 in order to obtain the information 240 about thegas concentration or the thermal diffusivity. For example, thepolynomial (e.g. polynomial y) may be obtained according toy=A0+A1*sigX+A2*sigX². Due to the polynomial formation of thecombination signal 230 by the evaluation arrangement 200, thecombination signal 230 may be corrected very easily and efficiently withrespect to possible pressure drift errors or temperature drift errors.

According to an embodiment, the evaluation arrangement 200 may beimplemented to multiply the polynomial of the combination signal 230with a correction term in order to obtain the information 240 about thegas concentration and/or the thermal diffusivity. The correction term ofthe combination signal 230 may depend on information about a pressureand on information about a temperature and may compensate a pressuredependence and temperature dependence, for example. In other words, thecorrection term may compensate a pressure drift and/or a temperaturedrift from the combination signal 230. Thus, a possible incorrectinterpretation by the evaluation arrangement 200 of the signals detectedby the thermal gas sensor 100 may be reduced.

According to an embodiment, the evaluation arrangement 200 may beimplemented to perform a calculation according to

$C = {{{pol}({sigX})} \cdot \left( {1 - {\left\lbrack \frac{f(p)}{{sigX} - {{const}\; 1}} \right\rbrack ~\left( {1 - \left\lbrack \frac{f(T)}{p - {{const}\; 2}} \right\rbrack} \right)}} \right)}$

in order to obtain the information C 240 about the gas concentration.sigX may be the combination signal 230, pol(sigX) may be a polynomial ofthe combination signal sigX 230, f(p) may be a function of a pressure p,const1 may be a constant, f(T) may be a function of the temperature T,and const2 may be a second constant. f(p) may be a function of apressure p measured in a surrounding area of the thermal gas sensor 100,and f(T) may be a function of a temperature T measured in a surroundingarea of the thermal gas sensor 100. The second term of themultiplication

$\left( {1 - {\left\lbrack \frac{f(p)}{{sigX} - {{const}\; 1}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{f(T)}{p - {{const}\; 2}} \right\rbrack} \right)}} \right)$

may also be understood as a correction term of the combination signal230. The correction term may depend on measuring conditions of the gassensor 100 (such as a surrounding pressure/measuring pressure, or asurrounding temperature/measuring temperature). Thus, the correctionterm may correct possible influences of a surrounding pressure or asurrounding temperature of the thermal gas sensor 100 on thedetermination of the information 240 about the gas concentration. Thus,a possible pressure drift or temperature drift may be suppressed.

According to an embodiment, the evaluation arrangement 200 may beimplemented to perform a calculation according to

${C\;\left\lbrack {{vol}\mspace{14mu} \%} \right\rbrack} = {{A.{y({sigX})}} \cdot \left( {1 - {\left\lbrack \frac{{B.{y(p)}} - {B.{ref}}}{{sigX} - {B.{ref}}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{{C.{y(T)}} - {C.{ref}}}{p - {C.{ref}}} \right\rbrack} \right)}} \right)}$

in order to obtain the information C 240 about the gas concentration. Inthe equation, sigX may be the combination signal 230, A.y(sigX) may be apolynomial of the combination signal sigX 230 (e.g. of the first order),B.y(p) may be a function of the pressure p (e.g. a polynomial function,such as of the second order), B.ref may be a constant, C.y(T) may be afunction of the temperature T (e.g. a polynomial function, such as ofthe second order), and C.ref may be a second constant. For example, thefunction B.y(p) may be a function of a pressure p measured in asurrounding area of the thermal gas sensor 100, and the function C.y(T)may be a function of a temperature T measured in a surrounding area ofthe thermal gas sensor 100. The second term

$\left( {1 - {\left\lbrack \frac{{B.{y(p)}} - {B.{ref}}}{{sigX} - {B.{ref}}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{{C.{y(T)}} - {C.{ref}}}{p - {C.{ref}}} \right\rbrack} \right)}} \right)$

of the multiplication for calculating the information C 240 about thegas concentration may define a correction term. In this case, forexample, the correction term may depend on the pressure p and thetemperature T. Thus, for example, B.y(p) may be a polynomial functiondependent on the pressure p, for example, as a result if which acorrection of possible pressure influences on the calculation of theinformation 240 about the gas concentration may be considered.Similarly, by forming the polynomial function C.y(T) as a function ofthe temperature T, a possible influence of the temperature T on thecalculation of the information 240 about the gas concentration may beconsidered very precisely. By forming the polynomial function as afunction of the pressure p and as a function of the temperature T, errorcorrections may be approximated very precisely, as a result of which theevaluation arrangement 200 may be implemented to determine theinformation 240 about the gas concentration very effectively and veryprecisely.

According to an embodiment, the evaluation arrangement 200 may beimplemented to consider a pressure and/or a temperature in a surroundingarea of the thermal gas sensor 100 when determining the information 240about the gas concentration and/or the thermal diffusivity. To this end,for example, the thermal gas sensor 100 may comprise pressure sensorsand temperature sensors with which it may detect the pressure and/or thetemperature in the surrounding area and transmit the same to theevaluation arrangement 200. Thus, for example, the evaluationarrangement 200 may consider and correct possible incorrect calculationsof the information 240 about the gas concentration and/or the thermaldiffusivity due to different pressure conditions and/or temperatureconditions in the surrounding area of the thermal gas sensor 100. Thus,the evaluation arrangement 200 may react to the pressure and/or thetemperature in the surrounding area of the thermal gas sensor 100 andmay accordingly very precisely determine the information 240 about thegas concentration and/or the thermal diffusivity.

According to an embodiment of the present invention, when determiningthe information 240 about the gas concentration and/or the thermaldiffusivity, the evaluation arrangement 200 may be implemented to use asinput quantities of a drift correction the combination signal 230,information about the temperature in a surrounding area of the thermalgas sensor 100, and information about a pressure in a surrounding areaof the thermal gas sensor 100, in order to obtain the information aboutthe gas concentration and/or the thermal diffusivity as a result of thedrift correction. Thus, for example, the drift correction may be appliedto the combination signal dependent on the information about thetemperature and the pressure in order to obtain the information 240about the gas concentration and/or thermal diffusivity. For example,apart from the three stated input variables (the combination signal, theinformation about the temperature, and the information about thepressure), the drift correction may obtain no further variables, but mayonly use previously obtained constants, such as those determined in thecontext of a calibration. In this case, the constants may be specificfor the thermal gas sensor 100 that is used. Thus, the evaluationarrangement 200 may be implemented to consider small differences betweenthermal gas sensors 100 when calculating the information 240 about thegas concentration and/or thermal diffusivity in order to obtain a veryprecise result (information 240). For example, the drift correction maycorrect a temperature drift and/or a pressure drift.

FIG. 1c shows a schematic illustration of an evaluation arrangement 200for a thermal gas sensor 100 with at least one heater 120 and twodetectors (a first detector 130 and a second detector 140). The firstdetector 130 may comprise a first distance 180 ₁ to the heater 120, andthe second detector 140 may comprise a second distance 180 ₂ to theheater 120. According to FIG. 1c , the first detector 130 and the seconddetector 140 comprise the same distance 180 ₁, 180 ₂ to the heater 120.However, it is also possible that the first distance 180 ₁ differs fromthe second distance 180 ₂. Thus, for example, the first detector 130 maybe arranged in a different distance to the heater 120 than the seconddetector 140. The evaluation arrangement 200 may be configured tocontrol (e.g. using a control unit 250 for controlling a heating power)a heating power, which may be applied to the heater 120, dependent onleast one sensor signal (e.g. a first sensor signal 210 and/or a secondsensor signal 220) from at least one of the detectors (e.g. the firstdetector 130 and/or the second detector 140) in order to bring the atleast one sensor signal 210, 220 into a predetermined value range.

For example, in order to analyze and/or further progress the at leastone sensor signal 210, 220 by the evaluation arrangement, it isadvantageous if the at least one sensor signal 210, 220 is brought intothe predetermined value range by the evaluation arrangement 200. Forexample, if the heating power is increased, an amplitude or a frequencyof the at least one sensor signal 210, 220 may also be increased, forexample. For example, this may be performed by the evaluationarrangement 200 if the at least one sensor signal 210, 220 is too smalland the predetermined value range is too large. Thus, the new sensorsignal 210, 220 may fill out, or be located in, the predetermined valuerange after the control of the heating power by the control unit 250.For example, the predetermined value range may depend on the componentsof the evaluation arrangement 200 that are used, e.g. an analog-digitalconverter (ADC). Thus, for example, the ADC may further process the atleast one sensor signal 210, 220 very efficiently if the at least onesensor signal 210, 220 is adapted in the predetermined value rangeadapted to the ADC (e.g. the ADC operating range).

The evaluation arrangement 200 may also be implemented to control theheating power of the heater 120 with the control unit 250 such that theheating power of the heater 120 is reduced. Through this, the at leastone sensor signal 210, 220 may also be reduced. For example, this may beadvantageous if the at least one sensor signal 210, 220 exceeds thepredetermined value range, i.e. is too large. Due to the fact that theevaluation arrangement 200 is implemented to control the heating powerof the heater 120 with the control unit 250, it is possible that, whenfurther processing the at least one sensor signal 210, 220 by exemplarycomponents of the evaluation arrangement 200, such as the ADC, no oronly little information of the at least one sensor signal 210, 220 islost.

According to an embodiment, the control unit 250 of the evaluationarrangement 200 may transmit a control signal 252 to the heater 120 forcontrolling the heating power of the heater 120. Additionally, thecontrol unit 250 may provide information 122 to the evaluationarrangement 200 about the controlled heating power of the heater 120.

The evaluation arrangement 200 may be configured to consider information122 about the heating power when deriving information 240 about a gasconcentration and/or thermal diffusivity from the at least one sensorsignal 210, 220. Thus, it is possible that the control unit 250 bringsthe sensor signal 210, 220 into the predetermined value range andadditionally considers the information 122 about the heating power inthe analysis since the at least one sensor signal 210, 220 depends onthe heating power. In addition, this evaluation arrangement 200 makes itpossible that one sensor signal, e.g. the first sensor signal 210 or thesecond sensor signal 220, may be sufficient to derive the information240 about the gas concentration and/or the thermal diffusivity of a gasor a fluid (e.g. of a gas or a gas mixture) with a certain accuracy. Ifthe first sensor signal 210 and the second sensor signal 220 as well asthe heating power 122 are used to derive the information 240, thedetermination of the information 240 is overdetermined, as a result ofwhich the information 240 may be determined very precisely by theevaluation arrangement 200. For example, if the first distance 180 ₁ ofthe first detector 130 to the heater 120 differs from the seconddistance 180 ₂ of the second detector 140, the information 240 about thegas concentration and/or the thermal diffusivity of a gas may just bederived from the first sensor signal 210 and the second sensor signal220, without using the information 122 about the heating power of theheater 120.

According to an embodiment, the evaluation arrangement 200 may alsoobtain the information 122 about the heating power from the thermal gassensor 100 instead of from the control unit 250.

According to an embodiment, the evaluation arrangement 200 may beimplemented to apply a periodic signal (e.g. the control signal 252) tothe heater 120. For example, the periodic signal may be a periodicsquare-wave signal or a sinusoidal signal. If the control signal 252,and therefore the heat dissipated to the gas to be analyzed by theheater 120, is a periodic signal, the first sensor signal 210 detectedby the first detector 130 and the second sensor signal 220 detected bythe second detector 140 may also be periodic. However, due to the firstdistance 180 ₁ and the second distance 180 ₂, the first sensor signal210 and/or the second sensor signal 220 may differ in phase with respectto the periodic signal of the heater 120, and may differ in amplitudewith respect to the periodic signal of the heater 120. For example, theevaluation arrangement 200 may use these differences to very preciselydetermine the information 240 about the gas concentration and/or thethermal diffusivity.

According to an embodiment, the evaluation arrangement 200 may beconfigured to switch the heating power applied to the heater 120 (e.g.by means of the control signal 252) between two values. Thus, forexample, a periodic square-wave signal may be applied to the heater 120.Thus, for example, the heater 120 may alternately transfer a firstheating power and a second heating power to the gas to be analyzed.

According to an embodiment, the evaluation arrangement 200 may beimplemented to control (e.g. with the control unit 250) an amplitude ofthe heating power such that a minimum value of the at least one sensorsignal 210, 220 and a maximum value of the at least one sensor signal210, 220 are in the predetermined value range. For example, if theamplitude of the heating power of the heater 120 is increased by thecontrol signal 252, the minimum value of the at least one sensor signal210, 220 may be decreased and the maximum value of the at least onesensor signal 210, 220 may be increased, for example. For example, ifthe amplitude of the heating power is decreased by the control signal252, the minimum value of the at least one sensor signal 210, 220 may beincreased and the maximum value of the at least one sensor signal 210,220 may be decreased.

According to an embodiment, the predetermined value range may depend ona value range of a component, such as an ADC, of the evaluationarrangement 200. Thus, for example, the predetermined value range may bedetermined dependent on a component value range (e.g. of a component ofthe evaluation arrangement 200). Thus, for example, the predeterminedvalue range may specify that the minimum value of the at least onesensor signal 210, 220 is to be in the range of 0% to 30%, 1% to 25%, or2% to 20% of the component value range, for example, and that themaximum value of the at least one sensor signal 210, 220 is to be in arange of 70% to 100%, 75% to 99%, or 80% to 98% of the component valuerange. Thus, for example, the predetermined value range may comprise alower value range in which the minimum value is to be located, and anupper value range in which the maximum value is to be located.

According to an embodiment, the evaluation arrangement 200 may beimplemented to set or adjust (e.g. with the control unit 250) anamplitude of the heating power such that an amplitude of the at leastone sensor signal 210, 220 is in a specified amplitude range. Forexample, if the at least one sensor signal 210, 220 comprises a periodicsinusoidal signal, the amplitude should be in the specified amplituderange at each point in time of the sensor signal. Here, the amplitude ofthe at least one sensor signal should utilize the full specifiedamplitude range. For example, the specified amplitude range maycomprise/be divided into an upper, center, and lower amplitude range.For the specified amplitude range to be utilize by the amplitude of theat least one sensor signal, a maximum amplitude of the at least onesensor signal should be in the upper range, and a minimum amplitudeshould be in the lower range, for example. For example, the specifiedamplitude range may depend on the component range. Thus, for example,the specified amplitude range may be determined such that the amplitudeof the at least one sensor signal utilizes at least 50%, or at least65%, or at least 75% of a component value range of an analog-digitalconverter, for example.

According to an embodiment, the evaluation arrangement 200 may beconfigured to set or adjust sampling times at which a sensor signal 210,220 may be sampled. For example, the sensor signal 210, 220 may beoptionally preprocessed by the evaluation arrangement 200 or the thermalgas sensor 100, and/or may be applied with a DC offset. According to anembodiment, it may be advantageous if the sensor signal 210, 220 issampled at a point in time of a maximum amplitude and at a point in timeof a minimum amplitude. For example, these two sampling times may be setor readjusted by the evaluation arrangement 200 if the evaluationarrangement 200 determines that the sampling times have been incorrectlyselected. By exactly setting the sampling times, it may be possible,e.g., that the evaluation arrangement may very easily determine a phasedifference or an amplitude difference between the first sensor signal210 and a heater signal (e.g. emitted by the heater 120 and controlledby the control signal 252) or between the second sensor 220 and theheater signal. By means of the very precise phase difference and/oramplitude differences, the evaluation arrangement 200 may very preciselydetermine, or derive, the information 240 about the gas concentrationand/or thermal diffusivity of the gas to be analyzed.

According to an embodiment, the evaluation arrangement 200 may beimplemented to set the sampling times such that a sampling, e.g., iscarried out with a phase difference of up to +/−2° at a point in time atwhich the sensor signal 210, 220 reaches a maximum value, and such thatthe sampling, e.g., is carried out with a phase difference of up to+/−2° at a point in time at which the sensor signal 210, 220 reaches aminimum value. For example, the maximum value may define a maximumamplitude of the sensor signal 210, 220, and the minimum value maydefine a minimum amplitude of the sensor signal 210, 220, as describedabove.

According to an embodiment, the evaluation means 200 may be implementedto combine a sensor signal 210, 220 from at least one of the detectors130, 140 with an offset signal generated by a digital-analog converterin order to obtain an input signal for the analog-digital converter. Theevaluation means 200 may be implemented to adjust the offset signal inorder to achieve that the input signal of the analog-digital converterremains within a specified range during a total period of the sensorsignal 210, 220. Thus, for example, the offset signal may be implementedto adapt the sensor signal 210, 220 such that the input signal that isin a component value range of the analog-digital converter is created.Thus, for example, the offset signal may be adjusted/adapted in order tobe able to react to different sensor signals 210, 220 from differentgases to be analyzed. Thus, for example, the offset signal may beconfigured to decrease a sensor signal 210, 220 that is too large sothat the resulting input signal is in the specified range. In addition,when the sensor signal 210, 220 is too small, the offset signal may beconfigured to increase the sensor signal 210, 220 so that an inputsignal that is in the specified range is created.

Thus, on the one hand, the evaluation arrangement 200 may be implementedto bring the amplitude of the sensor signal 210, 220 into thepredetermined value range by controlling the heating power, and tochange an offset of the sensor signal 210, 220 by combining the sensorsignal 210, 220 with the offset signal such that the sensor signal 210,220 is in a predetermined value range. This makes it possible that thesensor signal 210, 220 may be analyzed very precisely, and that veryprecise information 240 about the gas concentration and/or the thermaldiffusivity of the gas to be analyzed may therefore be determined by theevaluation arrangement 200.

According to an embodiment, the evaluation means 200 may be implementedto control the heating power only when a setting or adjustment of thesampling times is in a steady state and when an adjustment of the offsetsignal is in a steady state. A steady state may be understood such thatthe sampling times have been determined by the evaluation means 200 suchthat the sensor signal 210, 220 may be sampled at predefined events(such as a maximum amplitude (maximum value), a zero crossing, or aminimum amplitude (minimum value)). Similarly, the steady state maysignify that the offset signal has been adjusted such that the sensorsignal 210, 220 generates, upon combining the offset signal with thesensor signal 210, 220, an input signal that is in the specified range,and to therefore very precisely analyze the sensor signal 210, 220 bymeans of the evaluation arrangement, without or with only littleinformation losses. Thus, for example, pre-settings (such as thesampling times in the steady state, or the offset signal in the steadystate) may be determined by the evaluation means 200 so that, whencontrolling the heating power by means of the control unit 250, the newsensor signal 210, 220 may be very precisely analyzed with thepre-settings and, under certain circumstances, a new control of thesampling times, or the offset signal, is not needed anymore to derivethe information 240 about the gas concentration and/or the thermaldiffusivity from the sensor signal 210, 220.

According to an embodiment, the evaluation arrangement 200 may beimplemented to stop the control of the heating power (e.g. by means ofthe control unit 250), while the sampling times are set or adjustedand/or while the offset signal is adjusted. Thus, for example, it may beensured that there are no changes made to the sensor signal 210, 220while the sampling times and the offset signal are not yet in a steadystate. Thus, this may be ensure that the sensor signal 210, 220 may beanalyzed very precisely since the sampling times and the offset signalmay be determined very precisely with only a very small susceptibilityto errors or none at all.

According to an embodiment, the evaluation arrangement 200 may beimplemented to control a mean heating power or a maximum heating powerand also an amplitude of the heating power. Thus, for example, thecontrol unit 250 may transmit as a control signal 252 a new heatersignal for the heater 120 to the thermal gas sensor 100, wherein thecontrol signal comprises a changed mean heating power, maximum heatingpower, or amplitude of the heating power, for example. However, it isalso possible that the control signal 152 includes information statinghow the mean heating power, the maximum heating power, or the amplitudeof the heating power is to be changed by the thermal gas sensor for theheater 120.

FIG. 1d shows a schematic illustration of an evaluation arrangement 200for a thermal gas sensor 100 with at least one heater 120 and twodetectors (e.g. a first detector 130 and a second detector 140) arrangedin different distances (e.g. a first distance 180 ₁ and a seconddistance 180 ₂) to the heater 120. For example, the first detector 130may comprise the first distance 180 ₁ to the heater 120, and the seconddetector 140 may comprise the second distance 180 ₂ to the heater 120.The evaluation arrangement 200 may be implemented to apply a periodicsignal 260 with a specified period duration to the heater 120. In thiscase, for example, the periodic signal may a square-wave signal, animpulse signal with a known power, or a sinusoidal signal. Optionally,it may also be a sinusoidal signal with harmonics, or a triangularsignal. The periodic signal may also be referred to as a heater signal,and may be transferred in the form of heat from the heater 120 to thefirst detector 130 and/or the second detector 140 via a gas to beanalyzed. The transferred heat may be detected by the first detector 130as a first sensor signal 210, and by the second detector 140 as a secondsensor signal 220. The first sensor signal 210 and the second sensorsignal 220 may comprise a first periodic signal and a second periodicsignal, respectively, each comprising the specified period duration.This makes it possible that the gas to be analyzed may be analyzed veryprecisely with respect to its gas concentration and/or thermaldiffusivity by the thermal gas sensor 100, or the evaluation arrangement200. The evaluation arrangement 200 may be implemented to sample atleast one sensor signal (e.g. the first sensor signal 210 and/or thesecond sensor signal 220) from one of the detectors 130, 140 at threepoints in time (e.g. by means of a sampling means 270). For example, asecond sampling time may be time-shifted by 90° with respect to theperiod duration (e.g. with +/−2°) compared to a first sampling time.Thus, for example, the second sampling time may be time-shifted by 1/4period durations, 5/4 period durations, or by 9/4 period durationscompared to the first sampling time. A third sampling time may betime-shifted with respect by 180° to the period duration compared to thefirst sampling time, or by 90° compared to the second sampling time. Thefirst sampling time, the second sampling time, and the third samplingtime may comprise a tolerance of +/−2%. That is, for example, the thirdsampling time may be time-shifted by 1/2 period durations, 3/2 perioddurations, or by 5/2 period durations compared to the first samplingtime. Thus, the sensor signal 210, 220 may be sampled at preciselydefined locations, enabling to very precisely determine information 240about a gas concentration and/or thermal diffusivity from the sensorsignal 210, 220. The evaluation arrangement 200 may be implemented todetect, based on three sampling values that are based on sampling thesensor signal at the first sampling time, the second sampling time, andthird sampling time (e.g. performed by means of the sampling apparatus270), whether a first sampling value and a third sampling valueconstitute a maximum value and a minimum value of the sensor signal 210,220. For example, this may be carried out by the examination apparatus280. For example, the examination apparatus 280 may ignore a DC offsetand may therefore examine, apart from a DC offset, whether the firstsampling value constitutes a maximum value, and the third sampling valueconstitutes a minimum value of the sensor signal 210, 220, for example.Thus, for example, the second sampling time may be a “zero crossing” ofthe sensor signal 210, 220 and may also be considered by the examinationmeans 280.

The first sampling time, the second sampling time, and/or the thirdsampling time, as well as the first sensor signal 210 and the secondsensor signal 220 may be used to determine the information 240 about thegas concentration and/or the thermal diffusivity of a gas detected bythe thermal gas sensor 100. Optionally, the heater signal 122 may beadditionally used in the determination of the information 240. Thus, forexample, a phase difference between the first sensor signal 210 and thesecond sensor signal 220 as well as an amplitude difference between thefirst sensor signal 210 and the second sensor signal 220 may bedetermined based on the sampling time/sample values. Optionally, a phasedifference and/or an amplitude difference between the first sensorsignal 210 and the heater signal 122 and/or between the second sensorsignal 220 and the heater signal 122 may be determined. The information240 about the gas concentration and/or thermal diffusivity may bedetermined from the phase differences and amplitude differencesdetermined in such a way.

According to an embodiment, the evaluation arrangement 200 may beimplemented to change the sampling times dependent on identifyingwhether the first sample value and the third sample value constitute amaximum value and/or a minimum value of the sensor signal 210, 220. Forexample, this may be done by a sampling control means 290. Thus, forexample, new sampling times may be determined if the first sample valueand the third sample value do not correspond to a maximum value and/or aminimum value of the sensor signal 210, 220. Controlling the samplingtimes can ensure that the sample values correspond to predeterminedvalues. For example, if the examination means 280 determines that thereare deviations outside of a tolerance (e.g. +/−2°), the sampling timesmay be changed/readjusted by the sampling control means 290.

According to an embodiment, the evaluation arrangement 200 may beimplemented to set or adjust the sampling times such that the firstsample value constitutes a first extreme value of the sensor signal 210,220, e.g. a maximum value or minimum value, and such that the thirdsample value constitutes a second extreme value, e.g. the minimum valueor the maximum value of the sensor signal 210, 220. For example, thesecond sample value may constitute a mean value or DC component of thesensor signal 210, 220, e.g. a “zero crossing”.

According to an embodiment, the evaluation arrangement 200 may beconfigured to, when setting or adjusting the sampling times, considerinformation about a point in time when the sensor signal 210, 220 passesthrough a specified threshold value. For example, this point in time maybe the second point in time, e.g., that may constitute a DC component ormean value of the sensor signal 210, 220. Thus, for example, theexamination means 280 may use the second sampling time for examining thefirst sampling time and/or the second sampling time. Thus, if theexamination means 280 determines, based on the second sampling time,that the first sample value does not correspond to a maximum value orminimum value of the sensor signal 210, 220 and that the third samplevalue does not correspond to the minimum value or the maximum value ofthe sensor signal 210, 220, the sampling control apparatus 290 may setthe sampling times anew or readjust them. For example, the specifiedthreshold value may define a “zero crossing” (e.g. apart from a DCoffset).

According to an embodiment, the evaluation arrangement 200 may beimplemented to examine whether a second sample value at the secondsampling time is identical to a mean value of the sample value at thefirst sampling time and the third sample value at the third samplingtime, and to detect, dependent on the examination, whether the firstsample value and the third sample value represent a maximum value and aminimum value of the sensor signal. For example, the second sample valueshould be identical, with a tolerance of at most ±1%, to a differencebetween the first sample value and the third sample value or identicalto an mean value of the sample value and the second sample value. Ifthis is not the case, the examination apparatus 280 may detect that thesampling times have been selected incorrectly. Since the first samplevalue constitutes a first extreme value, and the third sample value,time-shifted by 180° with respect to the period duration, constitutes asecond extreme value of the sensor signal 210, 220, the second samplevalue may be located at exactly half the time between the first samplingtime and the second sampling time. Thus, the second sampling value maycorrespond to the mean value of the other two sampling values. Thus,this may constitute an efficient and exact method to examine the samplevalues with the help of the examination apparatus 280.

According to an embodiment, the evaluation arrangement 200 may beimplemented to apply a periodic square-wave signal 260 to the heater 120with a duty factor of 50%. However, it is also possible that theperiodic square-wave signal comprises a duty ratio in the range of 5% to50%, 8% to 48%, or 10% to 45%. The periodic square-wave signal 260applied to the heater 120 may comprise a tolerance of +/−2%. Accordingto an embodiment, the duty ratio indicates for a periodic sequence ofimpulses a ratio of an impulse duration to a period duration.

According to an embodiment, the evaluation arrangement 200 may beimplemented to combine a sensor signal 210, 220 with an offset signalgenerated by a digital-analog converter in order to obtain an inputsignal for an analog-digital converter. For example, the analog-digitalconverter may digitize the signal values (e.g. the first sample value,the second sample value, and/or the third sample value) present at thesampling times and uses this to sample the sensor signal 210, 220. Forexample, the sampling apparatus 270 may comprise the analog-digitalconverter.

According to an embodiment, the evaluation arrangement 200 may beconfigured to adjust the offset signals in order to achieve that theinput signal of the analog-digital converter remains within a specifiedrange during an entire period of the sensor signal 210, 220. Thus, forexample, the offset signal may change an offset of the sensor signal210, 220 such that an input signal that is in an operating range (e.g.the specified range) of the analog-digital converter is created, so thatno information of the sensor signal 210, 220 is lost in thedigitization, or so that an information loss is reduced. Thus, forexample, the sampling apparatus 270 may examine whether an input valueof the analog-digital converter exceeds a specified upper thresholdvalue, e.g. of the specified range, or falls below a specified lowerthreshold value, e.g. of the specified range. Accordingly, the samplingapparatus 270 may generate the offset signal that may be combined withthe sensor signal 210, 220 so that the input value, e.g. a value of theinput signal, remains in the specified range. The evaluation arrangement200 may be implemented to adjust the sampling times after adjusting theoffset signal and to again perform, after a change of the samplingtimes, an examination as to whether sample values obtained with thechanged setting of the sampling times are still within the specifiedrange. Thus, for example, the offset signal may be initially generatedfor the sensor signal 210, 220 by the evaluation arrangement 200, andsampling times may subsequently be determined, examined, and possiblyreadjusted by the sampling apparatus 270 (e.g. this may constitute atracking of the sampling times). After this tracking, new sample valuesthat may involve a repeated adjustment of the offset signal by theevaluation arrangement 200 may be created. Thus, for example, the offsetsignal and the sampling times may be alternately adjusted, or tracked,until the analog-digital converter may process the sensor signal 210,220, for example. Thus, at this point in time, the offset signal and thesampling times may be in a steady state.

For example, the settings of the sampling times changed by the samplingcontrol means 290 generate new sample values that may be considered asinput values of the analog-digital converter. For the input signal ofthe analog-digital converter to remain in the specified range, theoffset signal and the heating power of the heater 120 may be readjusted.For example, the offset signal may adapt an offset of the sensor signal210, 220, and the change of the heating power may adapt an amplitude ofthe sensor signal 210, 220, so that an input signal that is in thespecified range is created.

According to an embodiment, the evaluation arrangement 200 may beimplemented to control a heating power applied to the heater 120,dependent on at least one sensor signal 210, 220 from at least one ofthe detectors 130, 140 in order to bring the at least one sensor signal210, 220 into a predetermined value range. The evaluation arrangement200 may be implemented to consider information about the heating power(e.g. the heater signal 122) when deriving information 240 about a gasconcentration and/or thermal diffusivity from the sensor signal 210,220. Thus, for example, upon an increase of the heating power of theheater 120, the sensor signal 210, 220 may experience an increase of anamplitude of the sensor signal 210, 220, or, upon a reduction of theheating power, the at least one sensor signal 210, 220 may experience adecrease of an amplitude of the sensor signal 210, 220. Thus, forexample, the sensor signal 210, 220 may be brought into thepredetermined value range by controlling the heating power of the heater120.

In the following, embodiments of the thermal gas sensor and theevaluation arrangement are described based on further drawings.

1.1 Technological Variations for a Thermal Gas Sensor

FIG. 2a and FIG. 2b each show a schematic illustration of a gas sensor100 for measuring physical gas properties. The thermal gas sensor 100may comprise a thin-layer membrane 110 and a heating element 120, e.g.,that may be arranged as a self-supporting bridge structure on themembrane 110 between a first discontinuation area 160 of the membrane110 and a second discontinuation area 170 of the membrane 110. In thecase of a wire sensor (an example for the temperature sensor structures130, 140; cf. FIG. 2 and FIG. 3), a thickness of the thin-layer membrane110 (consisting of several base layers, sensor layers, and passivationlayers, for example) may be between 1-10 μm, for example. The heatingelement 120 may also be referred to as a heater. According to FIG. 2aand FIG. 2b , the entire first discontinuation area 160 may comprise adiscontinuation 162 of the membrane 110, and the entire seconddiscontinuation area 170 may comprise a discontinuation 172 of themembrane. Thus, the heating element 120 may be arranged in aself-supporting manner between the first discontinuation 162 and thesecond discontinuation 172. The first discontinuation 162 may be limitedby the heating element 120 and a first temperature sensor structure 130in the form of a self-supporting bridge structure. The seconddiscontinuation 172 may be limited by the heating element 120 and asecond temperature sensor structure 140, e.g., in the form of aself-supporting bridge structure. The first temperature sensor structure130 and/or the second temperature sensor structure may be a wire sensor,thermopiles, temperature-variable resistors or thermistors.

Optionally, the gas sensor 100 may comprise a first outerdiscontinuation 192 and a second outer discontinuation 194. Thus, forexample, the first thermal element structure 130 may be aself-supporting bridge structure between the first discontinuation 160and the second outer discontinuation 194, and the second thermal elementstructure 140 may be a self-supporting bridge structure between thesecond discontinuation 172 and the first outer discontinuation 192. Thefirst thermal element structure 130 may also be referred to as a firstdetector or a first sensor, the second thermal element structure 140 mayalso be referred to as a second sensor or a second detector.

A cross-section of the gas sensor 100 can be seen in the upper area ofFIG. 2a . For example, the gas sensor 100 includes a frame 150 made of acarrier material. For example, the frame 150 made of a carrier materialmay spread the membrane 110. According to an embodiment, the membrane110 may comprise a thickness 111 (e.g. an expansion perpendicular to asurface of the membrane 110 on which the first thermal element structure130, the second thermal element structure 140, and the heating element120 are arranged) in a range of 1 μm to 50 μm, 2 μm to 25 μm, or 3 μm to10 μm, e.g. 8 μm. According to an embodiment, the membrane 110 may berealized by recess 190 from the frame 150. Thus, for example, the recess190 may be selected such that a membrane 110 may be realized with thedesired thickness 111.

According to the embodiment in FIG. 2a and FIG. 2b , the recess 190 maybe implemented such that only the heating element 120, the first thermalelement structure 130, and the second thermal element structure 140remain spread out between the frame 150, for example. According to anembodiment, a surface of the membrane 110 on which the first thermalelement structure 130, the second thermal element structure 140, and theheating element 120 are arranged may comprise an expansion in a rangefrom 200×200 μm² to 5×5 mm², 500×500 μm² to 2000×2000 μm² or 800×800 μm²to 1200×1200 μm², wherein the expansion may be a square or rectangularexpansion. The gas sensor 100 may comprise a thickness 101 (e.g. inparallel to the thickness 111 of the membrane 110) in a range from 500nm to 5 mm, 1 μm to 1 mm, or from 200 μm to 600 μm, e.g. 400 μm. Anexpansion of the gas sensor 100 in parallel to the surface of themembrane 110 on which the heating element 120 is arranged may be in arange from 1×1 mm² to 1×1 cm², 1.5×1.5 mm² to 9×9 mm², or from 2×2 mm²to 8×8 mm², e.g. 6.5×2.5 mm².

According to an embodiment, the first thermal element structure 130, thesecond thermal element structure 140, and/or the heating element 120 maybe part of the membrane 110.

In order to measure a heat transport that depends on the gas type and/orthe gas mixture, a microchip (an example for the thermal gas sensor 100)with three fine bridge structures (e.g. the heating element 120, thefirst thermal element structure 130, and the second thermal elementstructure 140) that are spread out in a self-supporting manner between aframe and may be surrounded as micro wires by gas to be analyzed may beused. For example, the gas to be analyzed may be arranged in the firstdiscontinuation 162, the second discontinuation 172, the first outerdiscontinuation 192, and/or the second outer discontinuation 194. Acentral bridge structure may be implemented as the heater 120, and twodetector structures (e.g. the first thermal element structure 130 andthe second thermal element structure 140) arranged on both sides indifferent distances to the heater 120 may be used as temperature sensorsfor measuring a transfer response from the gas mixture.

For example, a periodic heat signal is applied to the center wire (theheating element 120), as a result of which heat is radiated by theheating element, for example. A heat transfer may take place via unknownheat transitions from the heater 120 into the gas to be analyzed, andfrom the gas into the sensor wire (e.g. into the first thermal elementstructure 130 and/or the second thermal element structure 140). The heattransfer detected in such a way by the first thermal element structure130 and/or the second thermal element structure 140 may be understood asa transfer response or as a sensor signal (e.g. a first sensor signaldetected by the first thermal element structure 130 and a second sensorsignal detected by the second thermal element structure 140) Bymeasuring a temperature response (e.g. the transfer response) with twoidentical sensors (e.g. the first thermal element structure 130 and/orthe second thermal element structure 140) in different distances to theheater 120, the unknown heat transitions in the measuring arrangementmay be eliminated, for example. The phase and amplitude of the twosensor signals may essentially depend on the heat transfer by the gas.

1.1.1 Example: the Gas Sensor 100 as an MEMS Wire Sensor (Evaluation ofa TCR (Temperature Coefficient of Resistance) at Detector Resistors(e.g. a Resistance of the First Thermal Element Structure 130 and/or theSecond Thermal Element Structure 140)) (Alternative Embodiment,Optionally Usable in Combination with the Signal Generation andEvaluation According to Section 1.2 and the Evaluation AlgorithmAccording to Section 1.3)

A first variation of the thermal gas sensor 100 may be built on thebasis of a silicon-on-insulator (SOI) wafer substrate. For example, itconsists of a microchip with self-supporting fine bridge structures madeof silicon micro wires (e.g. the first temperature sensor structure 130and the second temperature sensor structure 140) spread out in the gasspace to be analyzed. A center wire may be implemented as a heater 120,and two detector wires (e.g. the first temperature sensor structure 130and the second temperature sensor structure 140) may be used astemperature sensors on both sides of the heater in different distancesto the same (cf. FIG. 2a , FIG. 2b ).

For example, FIG. 2a shows an image of the MEMS wire sensor chip (thegas sensor 100) in a light microscope and FIG. 2b shows a close-up ofstructures in a scanning electron microscope.

FIG. 3 shows a schematic illustration of a silicon bridge 120/130/140that may be used for a heating element, a first thermal elementstructure, and/or a second thermal element structure of a gas sensor,for example. In other words, FIG. 3 shows a detail of a micro bridge(SEM, scanning electron microscope) of a thermal MEMS wire sensor (e.g.a gas sensor). For example, the illustrated silicon bridge 120/130/140may be manufactured in SOI technology. Thus, for example, a substrate orcarrier material of a frame 150 may comprise an oxide material 152, asilicon material 154, and an aluminum material 156. For example, inorder to realize the silicon bridge, the silicon material 154 may bepartially removed in order to realize cutouts 158 (e.g. trenches) in thecarrier material of the frame 150 and to therefore realize the siliconbridge 120/130/140. The silicon bridge 120/130/140 may be arranged onthe membrane 110 (e.g. consisting of the oxide material 152).

For example, the membrane 110 may comprise a first discontinuation area160/162 and a second discontinuation area 170/172. The firstdiscontinuation area 160/162 and the second discontinuation area 170/172comprise a discontinuation that may be a cavity, for example. Thus, themembrane 110 may comprise a first discontinuation 162 and a seconddiscontinuation 172 in which the gas to be analyzed may be arranged andwhere heat is transferred to the same from the silicon bridge120/130/140 if the silicon bridge constitutes a heating element 120, orwhere the same may transfer heat to the silicon bridge 120/130/140 ifthe silicon bridge 120/130/140 constitutes the first thermal elementstructure 130 and/or the second thermal element structure 140. Thesilicon bridge 120/130/140 may be contacted by the aluminum material156, as a result of which the aluminum material 156 may be used as abond pad, for example. For example, by means of the bond pad, anexcitatory heater signal may be applied to the heating element 120, orthe first thermal element structure 130 and/or the second thermalelement structure 140 may be read out (e.g. a first or second sensorsignal).

Advantages of the SOI Technology:

-   -   Crystalline resistive paths, the temperature coefficient of the        resistance (TCR) for the detectors (e.g. for the first thermal        element structure 130 and the second thermal element structure        140) may solely depend on the base doping of the wafer material        (in the active layer);    -   TCR of similar magnitude as in platinum at a high base        resistance of the resistors of the temperature detectors (e.g.        the first thermal element structure 130 and the second thermal        element structure 140) enables miniaturized sensor dimensions        (e.g. dimensions of the first thermal element structure 130 and        the second thermal element structure 140) due to short resistive        paths (e.g. from a frame side of the frame 150 to an opposite        frame side of the frame 150) of the bridge structures 120, 130,        140 (shorter than 1 mm) and, for the area of resistance        temperature detectors (RTD) (e.g. a first thermal element        structure 130 and the second thermal element structure 140),        comparably small temperature measuring errors due to        self-heating since, e.g., base resistance values larger than 8        kOhm may be used, which may need less than 360 μW of power input        during the measuring operation.    -   Heater resistance (e.g. of the heater 120) adaptable to a low        operating voltage (advantageously 3.3 v) by implantation;    -   Very homogenous distribution of the ohmic sensor resistance,        e.g. the resistance of the first thermal element structure 130        and/or the second thermal element structure 140, above the wafer        (e.g. the frame 150) in a very narrow process field, in        particular, the tolerances of the detector resistances (e.g.        sensor resistances) are determined, e.g., by tolerances of the        SOI material in an active layer (active layer, base doping, and        material thickness) as well as by the lateral structure accuracy        of the deep etching (Deep RIE).

Disadvantages of the SOI Technology:

-   -   Comparably expensive SOI substrate material when purchasing        wafers;    -   Often not available in desired specifications (wafer diameter,        material thickness of handle and active layer, doping of the        active layer);    -   Currently no passivation of the structures, under certain        circumstances, passivation leads to bimetal effects due to the        different material expansion of the layers upon heat input,        variation of the characteristic curve of the TCR;

1.1.2 Example: the Gas Sensor 100 as a MEMS Thermopile Sensor on aThin-Layer Membrane (Embodiment According to Aspect 1, Optionally Usablein Combination with the Signal Generation and Evaluation According toSection 1.2 and the Evaluation Algorithm According to Section 1.3)

FIG. 4 shows a schematic illustration of a gas sensor 100 on the leftside and a detailed view of the gas sensor 100 on the right side.

According to an embodiment, the gas sensor 100 may comprise a membrane110 and a heating element 120 that may be arranged on the membrane 110between a first discontinuation area 160 of the membrane 110 and asecond discontinuation area 170 of the membrane 110. The firstdiscontinuation area 160 may comprise a discontinuation 162, and thesecond discontinuation area 170 may comprise a discontinuation 172.

The first discontinuation 162 and/or the second discontinuation 172 maycomprise a longitudinal expansion in parallel to a direction of maximumexpansion of the heating element 120 (that may be referred to as aheater, for example), and may comprise a lateral expansion, e.g. in adirection perpendicular to a direction of maximum expansion of theheating element 120. According to FIG. 4, the first discontinuation 162may therefore have a larger lateral expansion than the seconddiscontinuation 172. In addition, according to FIG. 4, the firstdiscontinuation 162 and the second discontinuation 172 may comprise thesame longitudinal expansion. For example, the first discontinuation 162and the second discontinuation 172 comprise the longitudinal expansionthat is large enough that the first discontinuation 162 and the seconddiscontinuation 172 fully cover the area between the first thermalelement structure 130 and the second thermal element structure 140,respectively, and the heating element 120. Thus, for example, thelongitudinal expansion of the first discontinuation 162 and the seconddiscontinuation 172 extends along the entire length of the heatingelement 120. This avoids that a majority of the heat radiated by theheating element 120 is transported via the membrane 110. Thus, it may beachieved that a majority of the heat is transferred to the respectivethermal element structure 130, 140 via the gas arranged in the firstdiscontinuation 162 and in the second discontinuation 172.

For example, the first thermal element structure 130 may comprise adifferent distance to the heating element 120 than the second thermalelement structure 140. Thus, for example, according to FIG. 4, the firstthermal element structure 130 comprises a larger distance to the heatingelement 120 than the second thermal element structure 140. For example,the first thermal element structure 130 may detect a first heat transfer210 from the heating element 120 to the gas in the first discontinuation162, and from the gas to the first thermal element structure 130, andmay sense the same as a first sensor signal. For example, the secondthermal element structure 140 may detect a second heat transfer 220 fromthe heating element 120 to the gas in the second discontinuation 172,and from the gas to the second thermal element structure 140, andprovide the same as a second sensor signal. Due to the differentdistance of the first thermal element structure 130 and the secondthermal element structure 140 to the heating element 120, a differencesignal may be formed from the first sensor signal and the second sensorsignal, as a result of which unknown transitions (e.g. a transition fromthe heating element to the gas and/or from the gas to the respectivethermal element structure) may be calculated out, and therefore, the gassensor 100 mainly, or only, considers the heat transfer via the gas inthe first discontinuation 162 or the second discontinuation 172.

According to an embodiments, the heat sensor 100 may further comprise aframe 150 that may spread out the membrane 110. The first thermalelement structure 130 and the second thermal element structure 140 maybe arranged at least partially on the membrane 110 and at leastpartially on the frame 150. In this case, the first thermal elementstructure 130 and the second thermal element structure 140 may comprisehot ends 132, 142 that are arranged to face the heating element 120. Inaddition, the first thermal element structure 130 and the second thermalelement structure 140 may comprise cold ends 134, 144 that may bearranged on a side of the thermal element structure 130 and the secondthermal element structure 140, respectively, opposite the side with thehot ends 132, 142 and that are therefore arranged facing away from theheating element 120. Thus, for example, the hot ends 132, 142 may bearranged on the membrane 110, and the cold ends 134, 144 may be arrangedon the frame 150. In this case, for example, the frame 150 may comprisea different material than the membrane 110. Through this, for example, areference temperature may be applied to the cold ends 134, 144 by meansof the frame material of the frame 150, with respect to a temperaturemeasured by means of the hot ends 130, 142 and transferred from theheating element 120.

In other words, the left illustration of the gas sensor 100 mayconstitute a layout, and the right side of FIG. 4 may constitute animage of the gas sensor 100 (e.g. a MEMS membrane sensor) for measuringa gas type-dependent heat transport (embodiment according to aspect 1),for example. For example, FIG. 4 shows a variation of the gas sensor 100with a constant discontinuation (e.g. a first discontinuation 162 and asecond discontinuation 172) of a membrane 110. For example, the constantdiscontinuation 162, 172 causes a main part of a heat transport betweena heater 120 and the detectors (e.g. the first thermal element structure130 and the second thermal element structure 140), e.g., to occurcompulsory via the measuring gas volume enclosed between the twoelements, e.g., via the measuring gas arranged in the firstdiscontinuation 162 and in the second discontinuation 172.

For example, in order to reduce the process effort in the technologicalfabrication of the gas sensor 100 and to increase the sensitivity duringthe measurement of the gas type-dependent heat transport 210, 220, amicrochip may be realized on the basis of a thin-layer membrane 110 withheater structures 120 and thermopile structures 130, 140 (detectors),wherein the thin-layer membrane 110 may be etched out in a lateral areabetween the heater 120 and the detectors 130, 140.

Compared to a wire sensor (e.g. described in section 1.1.1), themembrane sensor (e.g. the gas sensor 100) only needs 1/3 of the heatenergy with an identical sensitivity for the gas concentration of abinary mixture. Same as with the wire sensor, the heater structure (e.g.the heating element 120) is located as a self-supporting fine bridgestructure centrally spread out in a measuring space of the gas to bedetected, for example. The two detector wires arranged on both sides(e.g.) in different distances to the heater 120 may be replaced by“thermopile” structures (e.g. of the first thermal element structure 130and/or the second thermal element structure 140) that may be located onlaterally spread out membrane surfaces (of the membrane 110) and mayreach up to the trench edge (e.g. an edge of the first discontinuation162 or the second discontinuation 172), for example.

For example, the cold ends 134, 144 of the thermopiles 130, 140 shoulddirectly contact the carrier material (e.g. of the frame 150) that mayhave a high thermal conductivity (e.g. silicon, approximately 150W/(*K)) and may serve as a heat sink (cooling body near roomtemperature). For example, the base membrane material (the material ofthe membrane 110), which electrically insulates the contacts from thesilicon, is located between the cold ends 134, 144 of the thermopilesand the silicon. However, since this layer is very thin, the heat fromthe thermopiles can be effectively transferred into the silicon. In thisway, the over-temperature (e.g. measured by means of the hot ends 132,142) may be measured as a direct difference to the room temperature(e.g. measured by means of the cold ends 134, 144). For example, ameasuring location for the temperature compensation is directlymechanically connected on or to the silicon chip (e.g. the frame 150).

In order to reduce a parasitic effect of the heat transport 210, 220between the heater 120 and the detector structures 130, 140 due to aheat conduction in the membrane material of the membrane 110, themembrane 110 may be consequently interrupted such that the heattransport 210, 220 of the heater 120 to the detectors 130, 140 may bemainly carried out via a shortest lateral distance, and therefore, e.g.,passes through a path across a volume of the measuring gas located inbetween (e.g. arranged in the first discontinuation 162 and the seconddiscontinuation 172). As a result, the gas type-dependent transferresponse (e.g. the first sensor signal and the second sensor signal) ofthe sensor 130/140 to periodic heat pulses of the heater 120 may besignificantly increased.

According to an embodiment, FIG. 5 shows on its left side a schematicillustration of the gas sensor 100 and on its right side an enlargeddetailed view of the gas sensor 100. The gas sensor 100 of FIG. 5 maycomprise the same features and functionalities as the gas sensor 100 ofFIG. 4, wherein the gas sensor 100 of FIG. 5 may differ from the gassensor 100 of FIG. 4 in a design of the first discontinuation area 160and/or the second discontinuation area 170. Thus, for example, the firstdiscontinuation area 160 of the gas sensor 100 of FIG. 5 may comprise amultitude of discontinuations 162 _(i), and the second discontinuationarea 170 may also comprise a multitude of discontinuations 172 _(i).Thus, for example, the index i of the discontinuations 162 _(i) of thefirst discontinuation area 160 of the gas sensor 100 may reach from 1 to23 since the first discontinuation area 160 may comprise 23discontinuations according to the embodiment in FIG. 5. For example, theindex i of the discontinuations 172 _(i) of the second discontinuationarea 170 of the gas sensor 100 may reach from 1 to 14 since the seconddiscontinuation area 170 may comprise 14 discontinuations according toan embodiment of FIG. 5. Optionally, the index i of the discontinuations162 _(i) and the discontinuations 172 _(i) may define a natural number,for example, wherein the index i indicates how many discontinuations 162_(i), 172 _(i) are present in a discontinuation area 160, 170.

The discontinuations 162 _(i), 172 _(i) may be arranged in the firstdiscontinuation area 160 and in the second discontinuation area 170,respectively, in rows in parallel to a direction of maximum expansion ofthe heating element 120, and the rows may additionally be arrangedoffset to each other. For example, this means that lateral ridges 112(e.g. extending in a direction perpendicular to a direction of maximumexpansion of the heating element 120, from the heating element 120 tothe respective thermal element structure 130, 140)—formed by membranematerial—of successive rows are arranged offset to each other. Forexample, this causes a parasitic heat conduction 114 a, 114 b in themembrane 110 to pass through as long a path as possible.

For example, the discontinuations 162 _(i), 172 _(i) are arranged suchthat a grid structure is created in the membrane 110, wherein a path ofa parasitic heat conduction 114 a, 114 b through the membrane 110 islonger than a direct path 210, 220. For example, a direct path 210, 220may be a straight path perpendicular to the heating element 120, fromthe heating element 120 to the respective thermal element structure 130,140, wherein the direct path 210, 220 may pass through a gas to beanalyzed that is arranged in the discontinuations 162 _(i), 172 _(i).For example, the path of the parasitic heat conduction 114 a, 114 bshould not extend in a straight line through the membrane 110, butshould form a winding path, as is illustrated in FIG. 5. For example,there should be no direct heat path across the membrane 110. This makesit possible that the first thermal element structure 130 and the secondthermal element structure 140 may detect a heat transfer from theheating element 120 via the direct path 210 and/or 220 and thatinfluences of a parasitic heat conduction 114 a, 114 b may be minimizedin the detection, as a result of which the gas may be analyzed veryprecisely.

For example, the discontinuations 162 _(i), 172 _(i) may be longitudinaldiscontinuations that may be perpendicular to a main direction of theheat conduction (e.g. the direct path 210, 220 from the heating element120 to the thermal element structures 130, 140) with a tolerance of+/−20°.

According to an embodiment, the discontinuations 162 _(i), 172 _(i) maybe rectangular cutouts with rounded corners. For example, they may alsobe referred to as a longitudinal hole, and they may also be oval holes,for example. In this case, the discontinuations 162 _(i), 172 _(i) maybe at least three times longer than they are wide. For example, thelength may be defined as a direction in parallel to a maximum expansionof the heating element 120, and the width may be defined as a directionperpendicular to the maximum expansion of the heating element 120. Dueto this feature, the path of the parasitic heat conduction 114 a, 114 bmay be realized to be very long, as a result of which a quality of thegas analysis by the gas sensor 100 may be increased.

According to an embodiment, the discontinuations 162 _(i), 172 _(i) inthe first discontinuation area 160 and the second discontinuation area170, respectively, may be arranged such that a distance 116 a, 116 bbetween the discontinuations 162 _(i), 172 _(i) corresponds to asmallest realizable structural width that results in a mechanicallydurable grid structure. For example, the distance 116 a, 116 b is awidth of ridges made of a membrane material over the membrane 110. Thesmaller the distance 116 a, 116 b is realized, the smaller a parasiticheat conduction 114 a, 114 b may be, as a result of which a quality of agas analysis by the gas sensor 100 may be increased. In this case, thedistance 116 a, 116 b should be selected such that the grid structuremembrane 110 created by the discontinuations 162 _(i), 172 _(i) ismechanically durable in order to ensure a high quality of the gasanalysis by the gas sensor 100.

In other words, FIG. 5 may illustrate a layout of an MEMS membranesensor (e.g. the gas sensor 100) for measuring the gas type-dependentheat transport (via the direct path 210, 220) (embodiment according toaspect 1), for example. Thus, the gas sensor 100 of FIG. 5 mayillustrate a variation having a grid structure made of the membranematerial of the membrane 110 in order to increase the mechanicalstability of the gas sensor 100. The geometrical shape of the grid maybe selected such that the parasitic heat conduction 114 a, 114 b has topass through as long a path as possible in the membrane material.

FIG. 5 shows a further embodiment of the gas sensor 100, showing a gridstructure between the heater elements 120 and the detector elements(e.g. of the first thermal element structure 130 and the second thermalelement structure 140) which is to improve the mechanical stability ofthe gas sensor 100 in the long-term operation. Such an arrangement maydecrease the gas type-dependent sensitivity of the thermal gas sensor100 since the heat conduction may now also occur in a parasitic manner114 a, 114 b via the grid ridges of the membrane material. Thus, a partof the heat energy periodically input into the heater 120 may betransported earlier to the detector structure (e.g. the first thermalelement structure 130 and/or the second thermal element structure 140)than the part of the heat energy that is transported through themeasuring gas via the shortest lateral distance 210, 220. Due to thethermal mass of the detectors (e.g. the first thermal element structure130 and/or the second thermal element structure 140) that may respond tothe periodic excitation as a low pass filter, for example, the twothermal wave runtimes (e.g. the parasitic heat conduction 114 a with theheat transfer via the direct path 210 and/or the parasitic heatconduction 114 b with the heat transfer via the direct path 220) arelooped together to a single sinusoidal detector signal (e.g. to a firstsensor signal or to a second sensor signal).

For example, the geometrical shape of the grid is selected such that theparasitic heat conduction 114 a, 114 b has to pass through as long apath as possible in the membrane material. For example, oval holes (e.g.the discontinuations 162 _(i), 172 _(i)) are located lateral to the maindirection of the heat conduction. For example, the aspect ratio of theoval holes is such that they are at least three times longer than theyare wide, the ridge width (e.g. the distance 116 a, 116 b) correspondsto the smallest realizable structural width that results in amechanically durable grid structure with the available layer technology,for example.

FIG. 6a , FIG. 6b , and FIG. 6c show schematic illustrations of furtherembodiments of a gas sensor 100. In this case, the gas sensor 100 ofFIG. 6a , FIG. 6b , and FIG. 6c may comprise the same features andfunctionalities as the gas sensor 100 of FIG. 4 and/or FIG. 5. There maybe differences between the gas sensors 100 in the first discontinuationarea 160 and the second discontinuation area 170 of the gas sensor 100.

Thus, for example, the gas sensor 100 of FIG. 6a may comprise eightdiscontinuations 162 _(i) in the first discontinuation area 160 andeight discontinuations 172 _(i) in the second discontinuation area 170.In this case, for example, the discontinuations 162 _(i) may comprise alarger lateral extension than the discontinuations 172 _(i). Inaddition, the discontinuations 162 _(i), 172 _(i) may comprise differentlongitudinal expansions within their discontinuation areas 160 and 170,respectively.

For example, the gas sensor 100 of FIG. 6b comprises a firstdiscontinuation area 160 with eight discontinuations 162 _(i) and asecond discontinuation area 170 with a continuous discontinuation 172.Thus, for example, in the variation of FIG. 6b , the variations of FIG.6a and/or FIG. 5 and FIG. 4 are combined with each other in thediscontinuation areas 160, 170.

For example, the gas sensor 100 of FIG. 6c comprises a firstdiscontinuation area 160 and a second discontinuation area 170 withseveral discontinuations 162 _(i), 172 _(i), wherein the firstdiscontinuation area 160 may comprise 23 discontinuations 162 _(i) andthe second discontinuation area 170 may comprise 14 discontinuations 172_(i), for example. In this case, for example, the continuations 162_(i), 172 _(i) of a discontinuation area 160 and 170, respectively, maycomprise the same lateral expansion and/or the same longitudinalexpansion. Optionally, it is also possible that the discontinuations 162_(i), 172 _(i) comprise only in rows the same longitudinal expansionand/or lateral expansion.

Thus, in other words, FIG. 6a , FIG. 6b , and FIG. 6c may illustratefurther layout variations of the MEMS membrane sensor (e.g. the gassensor 100), which differ in number and size of the perforations of themembrane (e.g. the discontinuations 162 _(i), 172 _(i)) (embodimentsaccording to aspect 1).

Advantages of the Thermopile Structures (e.g. the First Thermal ElementStructure 130 and/or the Second Thermal Element Structure 140) onMembrane Technology (Examples)

-   -   Simple 5-mask MEMS processed on cost-efficient substrates is        possible since the properties of the wafer material should be        specified only with respect to, e.g., thickness, surface quality        and, for structuring the trench, adapted base doping (in        contrast to the gas sensor on SOI structuring of a trench (e.g.        for the membrane 110).    -   For example, the structures (e.g. the heating element 120, the        membrane 110, the first thermal element structure 130, the        second thermal element structure 140) are passivated with        protective layers and provide better resistance against free        radicals that may be located in the measuring gas and that etch        the active sensor structures (e.g. the first thermal element        structure 130 and/or the second thermal element structure 140)        and therefore mechanically weaken or thermally change them.    -   For example, compared to the sensor on SOI substrate, the gas        sensor 100 on a thin-layer membrane 110 only needs a third of        the heating power to achieve the same gas sensitivity, the power        input is approximately 12 mW in contrast to 36 mW in the SOI        technology.    -   Instead of temperature-variable resistance structures (RTD),        thermopiles 130, 140 may be realized as detectors of a heat        distribution field in the measuring space: for example, the        electronic signal evaluation of the thermopiles 130, 140 is 0.6        μW, therefore almost powerless, whereas the detectors (e.g. the        first thermal element structure 130, the second thermal element        structure 140 of FIG. 2a , FIG. 2b , or FIG. 3) based on        resistance structures of the SOI technology need a current flow        for a stable signal generation, as a result of which a heating        power is applied into the detector, which is at approximately        140 μW and therefore low, however, it is 200 times larger        compared to the thermopile technology and contributes to the        self-heating of the RTD detectors and may therefore reduce the        gas selectivity in a parasitic way.

Disadvantages of the Membrane Technology:

-   -   Fine perforated membranes 110 may break in the production        process and in the long-term operation, an optimized design        (e.g. FIG. 4, FIG. 5, FIG. 6a , FIG. 6b , or FIG. 6c ) is        favorable.

1.1.3 Sensor Principle (Details Optional)

FIG. 7 illustrates a fundamental principle of the thermal sensor 100(the gas sensor may here also be referred to as a thermal sensor): Whatcan be clearly seen is the spatial separation between the heater 120 andthe sensor structures 130, 140 (the first temperature sensor structureand the second temperature sensor structure may here also be referred toas sensor structures, detector structures, sensors, temperature sensorsor detectors) with thermal coupling by means of the gas mixture to beanalyzed; and the measurement with the sensor structures 130, 140. Inthis case, the sensor structures 130, 140 may be arranged in differentdistances or in the same distance to the heater 120.

In other words, FIG. 7 shows a schematic illustration of a fundamentalsensor principle for a path 122 a, 122 b of the heat transport betweenthe heater 120 and the detectors 130, 140 via the gas to be measured.

The Heater 120 and the Sensors 130, 140 are Separated by a Medium

The heater 120 and the sensor(s) 130, 140 are arranged separately in themedium and are surrounded by the gas to be analyzed. For example, theheat flow 122 a, 122 b from the heater 120 to the temperature sensors130, 140 is carried out only via the gas itself.

Measurement in Several Distances

For example, the heat transport 122 a, 122 b is also carried out viaunknown heat transitions 122 a ₁, 122 b ₁ from the heater 120 into thegas to be analyzed, and via unknown heat transitions 122 a ₂, 122 b ₂from the gas into the sensor structure 130, 140. When measuring in twodistances 180 ₁, 180 ₂, the heat transitions 122 a ₁, 122 b ₁, 122 a ₂,122 b ₂ are almost identical. The difference of both sensor signalsessentially depends on the heat transfer by the medium itself.

Measurement in Identical Distances

Analogously to the measurement with several distances, in this casethere are also unknown heat transitions 122 a ₁, 122 b ₁, 122 a ₂, 122 b₂. A very precise gas analysis may also be performed by evaluating a sumof the two sensor signals and, under certain circumstances, the unknownheat transitions 122 a ₁, 122 b ₁, 122 a ₂, 122 b ₂ may also beconsidered in the analysis.

It is to be noted that, when measuring in several distances, a sumsignal may be evaluated as an alternative.

It is further to be noted that evaluating a sum signal is moreadvantageous than evaluating a difference signal since a signal-noisedistance of the difference signal is smaller than in the sum signal.

Optionally, a quotient of a difference signal and sum signal (which is acommon standardization) may be used for the evaluation. For example,this highlights the measuring effect more strongly as is the case ifonly the sum signal or only the difference signal is evaluated.

Electrical Analogy

An electrical analogy has been created (cf. FIG. 8, for example) inorder to identify and estimate the heat flows. Optimizing the heat lossis an essential factor in order to increase the sensitivity of thesensor 130, 140 without having to feed in too large of a heating power,e.g. via the heating element 120.

According to an embodiment, FIG. 8 comprises features andfunctionalities of the gas sensor 100 of FIG. 7. In other words, FIG. 8shows a schematic illustration of the heat transport at the gas sensor100. The heat transport from the heater 120 (temperature T_(H)) to thesensor 130, 140 (temperature T_(S)) essentially takes place via the gasto be measured.

1.2 Embodiment of the Gas Sensor in Operation: Signal Generation andSignal Evaluation on an Embedded System 1.2.1 Functional Principle(Details Optional)

With a sinusoidal heating power 122, there is a sinusoidal progressionof the sensor signals 210, 220 (e.g. FIG. 9, for example) that stronglydepends on the thermal properties of the gas surrounding the sensorstructures. By measuring the temperature of the heater 120 with twoidentical sensors 130, 140 in different distances 180 ₁, 180 ₂ to theheater 120, the unknown heat transition in the measuring arrangement maybe eliminated or reduced.

In the evaluation, emitted and received periodic temperature waves arecompared (cf. FIG. 9). A calibration of the signal 210, 220 through thephase shift 212, 222 between the heater and the sensors, for example,may be used to resolve the CO₂ content in the air as being 0.2 vol %,e.g. by means of the gas sensor. Since gases may be compressed andchange their density through pressure and temperature, the correspondingdrifts should be compensated.

FIG. 9 shows signals 210, 220 upon excitation with a sinusoidal heatingpower 122 in comparison for CO₂ and N₂. With the same heating power 122,the sensor signals 210, 220 received differ with respect to theiramplitude, offset and phase position. According to an embodiment, thesignals 210, 220 are difference signals of a signal of a first thermalelement structure and a second thermal element structure of the gassensor.

By evaluating further measuring quantities that the sensor provides, thethermal conductivity, the thermal diffusivity and, if the density of thegas is known, also the specific heat capacity may be determined—apossible approach to analyze unknown gas mixtures as well.

Through the structural difference of self-supporting bridge structurescompared to thin-layer membranes, parasitic thermal decoupling betweenthe heater and the detector elements is mostly achieved, and the signalquality is significantly increased. Due to the low thermal mass of theheater, it is possible to modulate the heater with frequencies of up to300 hertz since the heat may be quickly provided and dissipated.

1.2.2 Theoretical Consideration for Determining the Thermal Diffusivity(Details Optional)

In order to determine the thermal diffusivity at a sinusoidal heatingpower 122, a model according to [Baehr 2008] may be used to describe thepropagating temperature field.

The following equation describes the time-dependent (time t) atemperature propagation along the longitudinal axis x in a rod that hasa sinusoidal temperature applied at one end (mean value T_(m), amplitudeT_(A), angle frequency ω):

T(x,t)=T _(m) +T _(A) ·η·e ^(−k) ¹ ^(:x)·sin(2πf·t−(k ₁ ·x+ϵ))  (1)

When entering into the gaseous medium from the heater, the temperaturefield experiences the phase shift ϵ₀ and the attenuation η₀.

$\begin{matrix}{ɛ_{0} = {{\arctan \mspace{14mu} \frac{k}{1 + k}\mspace{14mu} {and}\mspace{14mu} \frac{1}{\eta_{0}}} = \sqrt{1 + {2k} + {2k^{2}}}}} & (2)\end{matrix}$

Dependent on the path x covered by the medium, the temperature fieldexperiences the phase shift ϵ(x)=k₁·x and the attenuation η(x)=e^(−k) ¹^(−x). The essential factor for the change of the path-dependent values,k₁, depends on the thermal diffusivity a, the angular frequency ω, andtherefore on the excitation frequency f, according to [Baehr 2008]:

$\begin{matrix}{k_{1} = {\sqrt{\frac{\omega}{2 \cdot a}} = \sqrt{\frac{\Pi \cdot f}{a}}}} & (3)\end{matrix}$

The factor for considering the influences in the heat transfer between asolid body and a gas results from the factor k₁, the heat transfercoefficient α, and the thermal conductivity λ:

$\begin{matrix}{k = {\frac{k_{1} \cdot \lambda}{\alpha} = {\frac{b}{\alpha} \cdot \sqrt{\Pi \cdot f}}}} & (3)\end{matrix}$

with the heat penetration coefficient b:

$\begin{matrix}{b = {\sqrt{\lambda \cdot c_{p} \cdot \rho} = \frac{\lambda}{\sqrt{\alpha}}}} & (4)\end{matrix}$

In order to determine the thermal diffusivity according to theabove-mentioned model, the evaluation of the phase shift is sufficient.The total phase shift in equation (1) amounts to:

Δφ=k ₁ ·x+ϵ ₀  (5)

When comparing two temperature measurements in two different distances,the constant heat transition effects cancel each other out:

Δφ(x ₂)−Δφ(x ₁)=(k ₁ ·x ₂+ϵ)−(k ₁ ·x ₁+ϵ)  (6)

Simplified with the differences Δφ₁₂=Δφ(x₂)−Δφ(x₁) and Δx₁₂=x₂−x₁

Δφ₁₂ =k ₁ ·Δx ₁₂  (7)

and with (3), the following results:

$\begin{matrix}{{\Delta\phi}_{12} = {{\sqrt{\frac{\pi {\cdot f}}{a}} \cdot \Delta}\; x_{12}}} & (8)\end{matrix}$

The following applies for the thermal diffusivity a (with angles in thecircular measure):

$\begin{matrix}{{a = \pi}{\cdot f \cdot \frac{\Delta x_{12}^{2}}{{\Delta\phi}_{12}^{2}}}} & (9)\end{matrix}$

If the phase shifts are available in degrees, the following applies forthe thermal diffusivity a:

$\begin{matrix}{a = {\frac{18{0^{\circ 2} \cdot f}}{\pi} \cdot \frac{\Delta x_{12}^{2}}{{\Delta\phi}_{12}^{2}}}} & (10)\end{matrix}$

The temperature wave oscillates harmonically at the same angularfrequency as its excitation and decays rapidly and strongly attenuatedwith increasing penetration depth in the medium, while the phase shiftsincreases. The penetration depth and wave length increase as theoscillation duration and thermal diffusivity of the medium increase.When considering the wavelength A of the temperature oscillation, whichresults from the distance between two measuring points x₁ and x₂ atwhich the phase angle differs by 2π, the penetration depth of thetemperature wave may be derived, where the temperature amplitude hasdecreased to the n-th part of its value at the entry point into themedium x=0. The following applies:

from e^(−2πx) ^(n) ^(/∧)=1/n, the following applies:

$\begin{matrix}{x_{n} = {{{\frac{}{2\pi} \cdot \ln}\mspace{11mu} n} = {\sqrt{\frac{a}{\pi \cdot f}}\mspace{14mu} \ln \mspace{11mu} n}}} & (11)\end{matrix}$

Thus, the attenuation of the amplitude is also a measure for the thermaldiffusivity of the medium.

1.2.3 Theoretical Consideration for Determining the Thermal Conductivity(Details Optional)

The thermal conductivity A of the medium is represented by the meantemperature distribution in the measuring space. Dependent on the meanheater temperature and the gas type and/or mixture concentration in thevolume of the measuring space, a mean temperature arises at thetemperature detectors, said mean temperature being in proportion to theheat flow that flows through the gaseous medium from the heater to thehousing wall via the detectors. The temperature of the heater and thatof the detectors have to be known to determine the thermal conductivity,e.g. with an appropriate calibration, it is sufficient to control adetector (advantageously the detector closer to the heater) to aconstant (over) temperature if the required mean heating energy isdetermined as a measure of the thermal conductivity.

According to [Simon 2002] and [Baar 2001], the fundamental principle formeasuring the thermal conductivity of gases is that an over temperatureabove the ambient temperature is generated in a flow-free measuringspace with a heater element (e.g. a hot wire or a “hot plate”) that isfree-standing in the gas. The heating power needed to maintain this overtemperature ΔT is the direct measure of the thermal conductivity A andmay be described with the following relationship:

P=λ·ΔT·G  (12)

wherein G represents the geometric constant of the arrangement. Thecondition for correct measurement is a stationary gas in the measuringspace, e.g. in a dead volume or behind a diffusion barrier, sinceconvective heat flow leads to a measuring error [Baar 2001]. Thesemeasuring errors are discussed in the literature, where methods that maymeasure the thermal conductivity in the presence of convective heat floware also proposed [IST AG 2011, 2013, 2015]. Furthermore, methods with aperiodic excitation of the heater are known, which may determine notonly the concentration of binary gas mixtures but also mixtures ofseveral components by Fourier analysis [Grien 2012].

1.2.4 Embedded Microcontroller, Electronic System and Software of theInventive Gas Sensor (Details Optional)

The object of the electronic system and signal evaluation is togenerate, e.g., a reliable measuring result that directly depends on thegas concentration with a miniaturized system that is as inexpensive aspossible. In addition, the inventive gas sensor should be usable in arespiratory gas monitor in which the carbon concentration in the airmixture may change very dynamically. Thus, the gas sensor should be ableto resolve changes in the gas composition in the respiratory cycle ofinspiration and expiration up to a rate of 60 strokes per minute. Thus,a fast evaluation of the sensor signals is desirable.

1.2.4.1 Hardware 1.2.4.1.1 Example: Heater Control of the Inventive GasSensor (Embodiments According to Aspect 3, Details Optional)

FIG. 10 shows an electric circuit diagram of a heater control for athermal gas sensor according to an embodiment of the present invention.For example, a CPU specifies a lower and upper heater voltage andswitches timers in a controlled manner back and forth between these twovalues. A CPU may measure the current heating current at certain pointsin time in order to calculate the heating power. In other words, FIG. 10illustrates a heater supply with a voltage specification and a currentmeasurement.

In contrast to the analogy in the above theoretically-considered analogyin the transfer of the principles for an attenuated oscillation to aheat transport phenomenon using the example of a sinusoidal heaterexcitation, (e.g.) a square-wave signal is generated on the developedmicrocontroller electronic system. Due to the timer structures in theprocessor, this signal may be generated much more precisely than asynthetic sinusoidal signal that would be output by the processor on itsdigital/analog (DA) port.

For example, 2 heater voltages are specified via a DA converter. This isdue to the fact that the DA converter is controlled via SPI, and thatthe point in time at which a new DA value is adopted may not bedetermined exactly with the selected processor component (CPU). However,this is a prerequisite in order to be able to determine the phaseposition of the sensor response. Thus, for example, one of the twovoltages is alternately applied to the heater amplifier via an analogswitch. For the steep switching edges to propagate less in the system,for example, they are smoothed out by a downstream low-pass filter. Theoperational amplifier (OP) circuit raises the voltage onto the voltagelevel needed by the heater. For example, a further OP compensates thevoltage drop at the current measurement resistor. Since the current ismeasured and the heater voltage is known, the heater power may becalculated. This is important because the heater resistance may changewith the temperature.

For example, a heater duty cycle of 50% may be used (wherein, e.g., aperiodic square-wave signal with a duty cycle of 50%+/−2% is applied tothe heater, for example).

Alternatively, shorter duty cycles may be used, e.g., in the range of 5. . . 50%.

In order to obtain the same power between a sinusoidal wave (offset atUpp/2, both half-waves in the positive range) and a square wave, a dutycycle of 42% is required for an “equivalent” square-wave signal or asquare-wave signal with the same power.

In some embodiments, adapting the heater power by controlling the dutycycle is not realized—this is more difficult on the MSP430, butinteresting when using more powerful microcontrollers: a fixed operatingvoltage may be used, and the duty cycle may be changed (a type of PWMcontrol).

In other words, it is optionally possible to set the (mean) heater powerby changing the duty cycle. Alternatively, the heater power may be setby changing the voltage level (of the voltage applied to the heater), orthe current level (of the current flowing through the heater, or theheating element). The two options may also be combined.

1.2.4.1.2 Example: Detector Signal Evaluation of the Gas Sensor (DetailsOptional)

FIG. 11 shows an electric circuit diagram of a detector signalevaluation of a thermal gas sensor according to an embodiment of thepresent invention. In this case, a first thermal element structure and asecond thermal element structure of the gas sensor may comprise thedetector signal evaluation illustrated in FIG. 11 in order to evaluate,in a respective detector signal (e.g. detected by means of the firstthermal element structure or the second thermal element structure, andmay also be referred to as a sensor signal herein), heat transferredfrom a heating element of the gas sensor to the first thermal elementstructure and the second thermal element structure via a gas to beanalyzed. According to an embodiment, FIG. 11 illustrates the detectorsignal evaluation of the sensor 1 (first thermal element structure). Inthis case, e.g., the detector signal evaluation is configured to receivea first input signal, e.g. a DAC signal CO2_S1_Win, from a CPU(magnifying glass function), and a second input signal, e.g. a detectorsignal CO2_Sensor1, and to provide a first output signal, e.g. anamplified detector signal CO2_S1_an, and a second output signal, e.g. acomparator signal for a phase evaluation CO2_S1_dig.

According to an embodiment, a CPU controls a heater such that anamplitude of the sensor signal remains within a ADC range. For example,the sensor signal is kept within the ADC boundaries via a magnifyingglass function. For example, a phase evaluation is carried out via thecomparator using the MSP430 timer structures (time structures).

A resistance change of the sensor wire (e.g. of the thermal elementstructure) is very low. For this reason, an amplifier having a highamplification factor is advantageous or needed. Since an absolute valueof an input voltage (e.g. of the sensor signals) depends on manyfactors, it is recommended to compensate for this value.

One possibility would be to use an alternating current (AC) amplifier.The disadvantage is that it causes an unknown phase shift.

Therefore, for example, a direct current (DC) amplifier has been used,which does not have any phase shift. In order to compensate for the DCcomponent of the signal, in an embodiment, the negative input terminalis raised to mean value of the detector signal at the differential inputof the operational amplifier (OP), and is actively tracked by means of asoftware controller, the digital-analog converter (DAC) of the processordirectly outputs this voltage. Due to the differential operation of thedifferential input at the OP, the DC components of the input voltagesare subtracted from each other, and only the AC component of the signalis amplified. To this end, according to an aspect, the (ADC) signalconverted from analog to digital is measured, and an examination as towhether it is within reasonable boundaries that may be detected by theADC is carried out. If the signal hits the upper or lower voltage limitof the OP, the DAC value is adapted accordingly. This results in anamplifier in which the amplified signal is continuously kept in theoptimum operating range or operating window, where the amplificationfactor at the OP may be increased by removing the DC component, a typeof “magnifying glass function”. The DAC value needed for thecompensation may be used as a further parameter for the evaluation, withwhich the absolute mean temperature may be determined, and the thermalconductivity of the gas mixture may be determined via the relationshipfrom equation (12).

In order to determine the phase position of the sensor signal, forexample, a Schmitt trigger was used. It is set such that it switchesshortly above or below the zero crossing of the sensor signal. Here, thesignal is steepest and therefore causes the smallest phase noise. Forexample, the DC component is removed via a capacitor. This enables aphase determination of the sensor response.

By using the internal timer structures of the processor (MSP430, TexasInstruments), a theoretical phase resolution of 0.009° is possible.However, this is not achieved due to noise of the circuit.

1.2.4.2 Example: Software (Details Optional; Functionalities Accordingto Aspects 3 and 4 are Described Together, but May be Used Separately)for the Gas Sensor

For example, the software has different tasks:

-   -   Setting the start values for the heater voltage, the sampling        times of the sensor signals and the start value for the DC        operating point (magnifying glass function).    -   Initially, for example, an attempt is made to find the DC        operating point. To this end, the DAC values of the two sensors        are set such that the sensor signal is centered in the ADC        range, for example.    -   Measuring the sensor voltage at certain points in time. In order        to determine the amplitude, the voltage is detected at the        assumed maximum and minimum. In order to recognize that the        sampling time has been selected incorrectly, another measurement        is made at the assumed “zero crossing”. If the sampling times        are correct, the following applies, for example:

$\frac{{U\mspace{11mu} \max} + {U\mspace{11mu} \min}}{2} = {U0}$

-   -   If the sampling times are incorrect, the above equation is no        longer correct. For example, the software may recognize from        this that the sampling times have to be adapted. For example,        the readjustment may be deactivated via software. It is only        carried out if the signal is within the ADC boundaries.    -   If the amplitude controller is active, an attempt is made, for        example, to keep the amplitude of the Sensor1 signal at a        certain target value. For example, the heater energy is adjusted        such that the S1 amplitude fills the ADC range by at least 3/4.        The controller may optionally be switched off via software. In        addition, for example, it is only active if sampling times or DC        offsets have not been changed. This optionally ensures that this        control loop is only active in the steady state.    -   Determining the phase position of the sensor signal with the        help of the Schmitt trigger circuit (optional). Dependent on the        setting, the calculation of the 3 sampling times of the sensor        signal for the next sampling period is also carried out here.    -   The ambient pressure and the temperature are detected via        further sensors (optional)

1.2.4.3 Example: Software Controller (Details Optional) for the GasSensor

FIG. 12 shows a schematic illustration of interleaved controllers of thesoftware for a thermal gas sensor according to an embodiment of thepresent invention.

Several interleaved controllers operate in the software. The innermostone is the DC operating point controller. For example, only if it is ina steady state (the DC offset did not have to be adapted), tracking thesampling times is carried out. In the amplitude control loop, e.g., theamplitude of S1 is kept constant—but only if, for example, the DC offsetand the sampling time did not have to be adapted. In the outer controlloop, the heating energy needed for adjusting the S1 amplitude may be(optionally) adjusted such that the thermal system may dynamically adaptitself to a large bandwidth of certain gas mixtures.

For determining the amplitude, for example, 3 A/D samples per sensorwire are needed: minimum at the lower peak, zero crossing, and maximumat the upper peak. For example, the process is as follows:

-   -   For example, all AD values are initially measured with the        current setting.    -   Now, for example, an examination as to whether the min/max A/D        values for S1 and S2 are in the valid range is carried out. If        this is not the case, the DC operating point of the amplifier is        readjusted (via DAC), and all further controllers are        temporarily switched off. Only when both sensor channels are        within the allowed operating range (A/D_(max)<3900, or        A/D_(min)>200, i.e. in the range from 5 . . . 95% of the A/D        range of 4096 digits), the further controllers become active        again.    -   To ensure a correct measurement of the amplitude, the A/D        conversion should be carried out at the correct time        (upper/lower peak, and at the zero crossing for verification).        Currently, for example, there are two ways to do this:        -   Through the A/D conversion itself: the time of the zero            crossing is expected in half the time between the two            measured times for the minimum and maximum peak of the A/D            values, i.e. (min+max)/2 should correspond to the A/D value            at the zero crossing. In case of deviations, the sampling            time for the next measurement is adapted. For example, a            deviation of approximately 0.625° (degrees) or 14.47 μs is            tolerated.        -   Through the comparator signal: since the comparator switches            at the time of the zero crossing of the sensor signal, for            example, the time at which the A/D measurements are to be            carried out may be determined: at the measuring value of the            switching time of the positive edge, 90° (or 2.0833 ms for            the upper peak), 180° (4.1666 ms for the zero crossing of            the negative edge), and 270° (6.2499 ms for the lower peak)            are added. Here, a deviation of 0.625° is also tolerated.    -   For example, only if both controllers (DC operating point and        phase) did not require a change of control values, and were        therefore in the steady state, then the amplitude controller        will take effect. It readjusts the heater value such that the        desired amplitude of S1 is achieved.

FIG. 13a shows a block diagram illustrating the control and tracking ofthe DC operating points of the two detector amplifiers according to anembodiment of the present invention. FIG. 13b shows a block diagramillustrating the tracking of the sampling times for the amplitudemeasurements of the detectors signals and S1 amplitude controllers. Ifall controllers are tuned, for example, the gas mixture is evaluatedwith the measured values for the amplitude and phase of the detectors.

According to an embodiment, FIGS. 13a and 13b may be considered to beone block diagram, where FIG. 13b is connected to FIG. 13a via the block“tracking the sampling times”.

1.2.4.4 Example: Timing Table (Details Optional) for the Gas Sensor

For example, the ADC measuring times at which the analog-digitalconverter of the microcontroller measures the current consumption of theheater and the detector voltages (an example for the sensor signals) aredefined in a timing table of the software extending across two heaterpulse periods. According to an embodiment, these two periods are needed,e.g., since only one timer is available on the processor used for thevariable ADC control. If the heater is operated at 120 Hz, all measuringvalues relevant for the gas mixture evaluation are obtained after 2periods, i.e. with a frequency of 60 Hz. Since the pulse shape of theheater is stable across the period, the input heater current may bemeasured at fixed times: at 45° for the peak value and at 170° for thelower heat current value (generally zero). The respective 3 ADCmeasuring values per detector (upper and lower peak, and zero crossing)are expected as variable measuring values in time windows that aredefined in the timing table:

-   -   ADC_SENSOR1:        -   CO2-S1-min: 33.6° . . . 123.6° (778 μs . . . 2861 μs)        -   CO2-S1-Null: 123.6° . . . 213.6° (2861 μs . . . 4944 μs)        -   CO2-S1-max: 213.6° . . . 303.6° (4944 μs . . . 7028 μs)    -   ADC_SENSOR2:        -   CO2-52-min: 68.6°-141.4° (1588 μs . . . 3273 μs)        -   CO2-S2-Null: 158.6°-231.4° (3671 μs . . . 5356 μs)        -   CO2-S2-max: 248.6° . . . 321.4° (5755 μs . . . 7440 μs)

1.3 Example: Evaluation Algorithm for Calibration with Respect to a GasMixture with Drift Correction for Gas Pressure and Gas Temperature (e.g.According to Aspect 2; Details Optional) of a Gas Sensor 1.3.1Measurements in Gas Mixtures 1.3.1.1 Binary Mixture

FIG. 14 exemplarily shows a CO₂ dependence of the sensor in the phasesignal at a constant temperature and constant pressure. Here, forexample, three phase shifts are illustrated; a phase differenceD1-Hz.dPhi (red) between the heater and the detector 1, with a distanceof 200 μm, a phase difference D2-Hz.dPhi (blue) between the heater andthe detector 2, with a distance of 300 μm, and a phase differenceD2-D1.dPhi (green, right y axis) between the detector 2 and the detector1. According to an embodiment, FIG. 14 illustrates phase shifts betweenthe heater and the detectors detectors for (0 . . . 5) vol % of CO₂ inthe air at a pressure of p=1010 mbar, a temperature of T_(amp)=24° C.,and a heating power of P=(15±12.5) mW at a frequency of f=120 Hz.

FIG. 15 illustrates exemplarily measured amplitudes at the detectors D1and D2 and a sum signal of the amplitudes formed relative to the heateramplitude, about the CO₂ dependence of the sensor. Here, for example,the amplitude D1.Uss (red) at the detector 1 and the amplitude D2.Uss(blue) at the detector 2 are illustrated. For example, at an increase ofthe CO₂ concentration, i.e. at an increase of the thermal diffusivity inthe gas mixture, the two amplitude signals fall off. By forming adifference of the heater amplitude and the sum of the detectoramplitudes, the relative amplitude signalsigUss=2*Hz.Uss-(D1.Uss+D2.Uss) (green, right y axis) will increase withan increase of the CO₂ content in the gas mixture, for example.According to an embodiment, FIG. 15 illustrates the amplitudes of thedetectors for (0 . . . 5) vol % of CO₂ in the air at a pressure ofp=1010 mbar, a temperature of T_(amp)=24° C., and a heating power ofP=(15±12.5) mW at a frequency of f=120 Hz.

1.3.1.2 Pressure Dependence

A sensor signal may depend strongly on the pressure and the temperature.To correctly determine the gas properties, the cross-effects shouldtherefore be known and corrected by the algorithms. For example, FIG. 16illustrates the cross-sensitivity of the sensor signal in the air withrespect to the absolute pressure and for different temperatures. What isexemplarily illustrated is the cross-sensitivity of a phase shift D2−D1between the detectors D2-D1 (e.g. between the first thermal elementstructure D1 and the second thermal element structure D2) for the airwith respect to a pressure p=(910 . . . 1110) mbar across differenttemperatures T_(amp)=(18 . . . 28°) C. in the air at a heating power ofP=(15±12.5) mW with a frequency of f=120 Hz.

The pressure influence shows a linear relationship, the temperatureinfluence shows a square relationship, as theoretically calculated. Bothcross-sensitivities are in the order of magnitude of the signal for thegas concentration.

1.3.1.3 Heating Power and Frequency Dependence

FIG. 17a shows an illustration of a sensor signal for a phase across thefrequency in a measurement in CO₂. In other words, FIG. 17a shows adiagram of a phase shift in 100% CO₂ as a function of the frequency. Thephase goes into saturation.

FIG. 17b shows an illustration of a sensor signal for an amplitudeacross the frequency in a measurement in CO₂. In other words, FIG. 17bshows a diagram of the amplitude in 100% CO₂ as a function of thefrequency. The amplitude decreases towards zero.

Compared to air, the heating power should be reduced in measurements infuel gases so that the system does not exceed its A/D range. The heatingpower variation has shown that it makes sense in practice to operate thesystem with the largest possible sensor amplitude and to thereforeobtain more stable signals, as compared to setting the heating power toa minimum, where the sample gas is less thermally influenced, but thesignal-to-noise distance also decreases. The heating energy periodicallyintroduced into the sensor has to be able to leave the sample volumewithin this period, for example, so that it does not heat upcontinuously. For example, a peak heating power of approximately 26 mWat 120 Hz was specified in three measurement systems.

The sensor behavior constitutes an ideal low pass filter of the 1storder, there are no overtone spectral components in the sensor signal.For this reason, actively sweeping through a frequency spectrum does notyield additional information. Thus, it was decided to operate the sensorat a fixed frequency, the effort with respect to electronics for thissystem could be reduced, the measuring time until a secured value isobtained is significantly shorter (all optional).

The higher the excitation frequency at the heater, the less energy maybe transferred between the heater and the detector via the gas, sincethe thermal masses of the sensor itself limit the transfer speedsbetween the solid body and the gas. The amplitude decreases withincreasing frequency up to a disappearing signal towards zero (cf. FIG.17b ), the phase shift saturates itself to a maximum (cf. FIG. 17a ).

Forming an optimum of a phase resolution, a phase difference and anamplitude for different gas mixtures resulted in the best phase responseat a frequency of, e.g., 120 Hz at a heating power of 26 mW for themicro sensor wire, and of 160 Hz at approximately 8 mW for the MEMSthermopile sensor on a thin-layer membrane (details optional).

1.3.1.4 Fuel Gas Mixtures

Different gas compositions were examined at a measuring station. FIG. 18shows a change of a phase signal of a sensor for methane with increasingaddition of nitrogen as a nearly linear behavior. For example, what isillustrated is the phase signal as a function of the nitrogenconcentration in methane as a phase difference D1-Hz.dPhi (red) betweenthe heater and the detector 1, with a distance of 200 μm, a phasedifference D2-Hz.dPhi (blue) between the heater and the detector 2, witha distance of 300 μm, and a phase difference D2-D1.dPhi (green, right yaxis) between the detector 2 and the detector 1. Here, according to anembodiment, the phase shift between heater-detectors is illustrated for(0 . . . 30) vol % of N₂ in methane at a pressure of p=990 mbar, atemperature T_(amp)=21° C., and a heating power of P=(13±12.5) mW at afrequency of f=120 Hz in FIG. 18.

FIG. 19 shows a diagram of the amplitude D1.Uss (red) detected by meansof the first detector, and the amplitude D2.Uss (blue) detected by meansof the second detector. Here, according to an embodiment, the amplitudesof the detectors are illustrated for (0 . . . 30) vol % of N₂ in methaneat a pressure of p=990 mbar, a temperature T_(amp)=21° C., and a heatingpower of P=(13±12.5) mW at a frequency of f=120 Hz in FIG. 19. Bothamplitude signals D1.Uss and D2.Uss fall off with an increase of the N₂concentration in methane, i.e. when decreasing the thermal diffusivityin the gas mixture, for example. By forming a difference of the heateramplitude and the sum of the detector amplitudes, the relative amplitudesignal sigUss=2*Hz.Uss−(D1.Uss+D2.Uss) (green, right y axis) increaseswith an increase of the N₂ concentration, for example.

FIG. 20 shows a diagram of a calculated sensor signal sigX (an examplefor a combination signal of the gas sensor) from a phase and anamplitude for different fuel gas mixtures. Thus, FIG. 20 shows thesensor signal (an example for a combination signal of the gas sensor)for different fuel gases and their mixtures: methane, ethane, andpropane, as well as the mixtures: methane95-ethane05,methane93-ethane05-CO202, methane91-ethane05-CO204,methane91-ethane05-CO202-propane02, methane90-ethane10 and natural gas-L(the 2-digit numbers indicate the proportion of gas components inpercent by volume). Methane, ethane and propane differ significantlyfrom one another, but the methane mixtures also differ from one anotherwith components of 2 vol % to 10 vol % of different gases. According toan embodiment, FIG. 20 illustrates the sensor signal for different fuelgases at a pressure of p=1001 mbar, a temperature T_(amp)=26° C., and aheating power of P=(13±12.5) mW at a frequency of f=120 Hz.

1.3.1.5 Findings from the Measurements in Gas Mixtures

The sensor signal shows strong pressure and temperature dependencies. Inorder to correctly determine the gas properties of a known mixture witha traceability to standard conditions and the comparison from tables,the cross-effects should therefore be known and corrected, for example.The pressure influence shows a linear relationship, the temperatureinfluence shows a square relationship. Both cross-sensitivities are inthe order of magnitude of the signal for the gas concentration.

1.3.2 Example: Method for the Calibration to a Gas Mixture with a DriftCorrection with Respect to a Gas Pressure and a Gas Temperature (e.g.According to Aspect 2, Details Optional) for a Gas Sensor 1.3.2.1 SumSignal of Phase and Amplitude (Example)

A combination of a phase/amplitude measurement has been shown to be aparticularly stable sensor signal (combination signal). For example,both signals are weighted with the aid of separate constants and addedand therefore combined to form a single sensor signal, for example:

sigX=sigUss*Ka+sigPhi*Kp  (13)

wherein sigX represents the calculated sum signal, sigUss represents therelative amplitude signal, and sigPhi represents the added phase signalof both detectors. The factors Ka and Kp are constants with which bothpartial signals are multiplied. For example, when converting theamplitude signal into mV, Ka=1/3500, and when converting the phasesignal into degrees, for example, Kp=1/276 for CO₂ air mixtures up to 30vol % of CO₂.

For example, the added phase signal sigPhi is formed from the sum of thetwo phase differences for the runtimes between the increasing edge ofthe heater impulse and the increasing edges at the detectors. Forexample, the following applies:

sigPhi=(D1−Hz)·phi+(D2−Hz)·phi  (14)

wherein (D1-Hz).phi and (D2-Hz).phi are to constitute the phasedifferences between the heater and the detectors.

As can be seen in FIG. 14, the phase difference between the heater andthe detectors increases with increasing CO₂ concentration, i.e. withincreasing thermal diffusivity, however, the two amplitudes at thedetectors fall off with increasing thermal diffusivity (FIG. 15).

For example, the relative amplitude signal becomes increasing with anincrease of the CO₂ content in the gas mixture due to a differenceformation of the heater amplitude and the sum of the detectoramplitudes:

sigUss=2*Hz.Uss−(D1.Uss+D2.Uss)  (15)

For example, the signal sigX calculated from the phase and theamplitudes is in the range between (1.7 . . . 2.0) for (0 . . . 6) vol %of CO₂, for example. The device (e.g. the gas sensor) was measured in atemperature range between (16 . . . 28°) C. and in a barometric pressurefield between (900 . . . 1200) mbar.

1.3.2.2 Drift Correction Via Polynomial Compensation (Details Optional)

When calibrating the sensor to a known gas mixture, the strong pressureand temperature dependence of the sensor signal should be compensatedfor in order to be able to infer a gas concentration from the measuringvalue.

For example, this results in a 4-dimensional vector field (matrix)consisting of a gas concentration (CO2 [vol %]), the sensor signal sigX(the sum signal of the phase and amplitude), the pressure drift and thetemperature drift. It is noticeable that the individual graphs in thediagram of FIG. 21 showing the dependence between the gas concentrationand the temperature signal, which each stand for a constant ambientpressure or a constant temperature, are shifted in parallel to eachother. If a mean graph is now formed from all the parallel shiftedcharacteristic curves, a normalized relationship of the signal isobtained for a mean temperature and a mean pressure (cf. red line 230 ain FIG. 21).

FIG. 21 shows the matrix of the measuring data of a variation of a gasconcentration of (0 . . . 5) vol % of CO₂ in nitrogen in a pressurerange of (900 . . . 1200) mbar, and in a temperature range of (16 . . .28°) C. With the aid of a pressure-dependent polynomial function, thegreen line 230 b of the calibration curve can be shifted towards acurrent operating pressure. The red line 230 a corresponds to the meanof all blue lines 230 ₁ to 230 ₁₆, and is a characteristic curve of thesensor signal for the gas concentration normalized to a mean temperatureand a mean pressure.

When plotting the characteristic curves of the sensor signal sigX fromthe measured variation are applied for each temperature and a mean gasconcentration across the pressure (cf. FIG. 22), a set of curves ofstraight lines shifted in parallel to each other is obtained as well.Higher pressures and a cold gas, i.e. gas molecules that are closer toeach other, lead to a higher sensor signal, low pressures and a warm gasresults in a low signal sigX.

Thus, FIG. 22 shows a pressure dependence of the sensor signal sigX fora mean fixed gas concentration, a set of curves of differenttemperatures. The lowest line 230 ₁ describes the relationship at thehighest temperature of 28° C. in the variation, and the highest line2307 illustrates the pressure dependence of the signal at 16° C.

If a horizontal line is placed into the parallel set of lines in FIG. 22for a fixed mean sensor signal, wherein said horizontal line intersectsall lines of the set of curves, the relationship between gas pressureand gas temperature of FIG. 23 is obtained.

FIG. 23 shows a slightly square relationship between a gas pressure anda gas temperature (for a mean gas concentration and a mean sensor signalsigX).

1.3.2.3 Determination of a Regression Constant (Details Optional)

When calibrating the gas sensor to a specific gas mixture, regressionsare formed from the variation matrix in succession through theabove-described relationships. Regression level A describes therelationship between the gas concentration of the calibration referenceand the sensor signal sigX. The individual curve per pressure andtemperature are each approximated in a square regression according tothe form: y=A.c0+A.c1*sigX+A.c2*sigX{circumflex over ( )}2. Since theincrease of all curves is approximately constant and the squarecoefficient c2 goes towards zero, the mean value is formed from allvalues for the coefficients A.c0, A.c1 and A.c2, the centralcharacteristic curve 230 a illustrated in red in FIG. 21 is obtainedacross the entire measuring value variation 230 ₁ to 230 ₁₆. This has tobe shifted on the x axis according to the drift influence of thepressure. Due to the pressure-dependent sigX₀=f(p), the associatedoffset A.c0 is sought, which is inserted into the equation of theregression plane A.

The regression plane B describes the pressure drift of the sensor signalsigX. The offset A.c0 is again calculated as a function of the pressuredrift: A.c0=sigX.y₀−B.c1*pressure.x₀−B.c2*pressure.x₀{circumflex over( )}2. If sigX.y₀=0, the equation is simplified to:A.c0=−(B.c1*pressure.x₀+B.c2*pressure.x₀{circumflex over ( )}2). The(now) pressure-dependent polynomial coefficient A.c0=f(p) is replaced inthe regression equation of the plane A (substituted), for example.

For example, the determined pressure-dependent offset for the polynomialof the regression plane A is calculated from the cosine relationship ofthe angle relationship between the offset and the increase with:A.c1=A.c0/sigX₀; sigX₀=f(p) and A.c0=(−1)*sigX*A.c1. With polynomials ofa higher order, the 1st derivative of the curve should be formed, andthe slope in the reference point should be calculated therefrom.

TABLE 1 polynomial coefficients of the three regression planes(examples) Reference to the previous plane (center Polynomialcoefficients Coefficient of the varia- Regressions plane c0 c1 c2 ofdetermination tion range) A Signal-to-CO2 −266.153759 144.315423 00.999258 0 B Pressure to signal 0.94394 0.001017 −1.50E−07 0.9999291.843335 shift C Temperature-to- 884.519093 7.844777 −0.023415 0.9983 1050 pressure shift

1.3.2.4 Converting the Signal to a CO2 Value (Example: Details Optional)

For example, the value for the gas concentration calculated from thepolynomial of the regression plane A is corrected by the pressure andtemperature drift:

$\begin{matrix}{{C{O_{2}\left\lbrack {{vol}\mspace{14mu} \%} \right\rbrack}} = {{A.{y({sigX})}} \cdot \left( {1 - {\left\lbrack \frac{{B.{y(p)}} - {B.{ref}}}{{sigX} - {B.{ref}}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{{C.{y(T)}} - {C.{ref}}}{p - {C.{ref}}} \right\rbrack} \right)}} \right)}} & (16)\end{matrix}$

wherein A.y(sigX), B.y(p) and C.y(T) correspond to the respective fullpolynomials for the measuring signal, the gas pressure, and the gastemperature.

If the fixed references constituting the geometric center of thevariation range are inserted into the equation, and the polynomials areresolved accordingly, the following equation results. WithB.ref=B.y(c.ref), the following applies:

$\begin{matrix}{{C{O_{2}\left\lbrack {{vol}\mspace{14mu} \%} \right\rbrack}} = {{A.{y({sigX})}} \cdot \left( {1 - {\left\lbrack \frac{{{B.c}\; {1 \cdot \left( {p - {C.{ref}}} \right)}} + {{B.c}\; {2 \cdot \left( {p^{2} - {C.{ref}^{2}}} \right)}}}{{sigX} - {B.{y\left( {C.{ref}} \right)}}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{{C.{y(T)}} - {C.{ref}}}{p - {C.{ref}}} \right\rbrack} \right)}} \right)}} & (17)\end{matrix}$

If C.ref=1050 mbar is inserted, the following applies:

$\begin{matrix}{{C{O_{2}\left\lbrack {{vol}\mspace{14mu} \%} \right\rbrack}} = {{A.{y({sigX})}} \cdot \left( {1 - {\left\lbrack \frac{{{B.c}\; {1 \cdot \left( {p - 1050} \right)}} + {{B.c}\; {2 \cdot \left( {p^{2} - 1050^{2}} \right)}}}{{sigX} - {B.{y(1050)}}} \right\rbrack \cdot \left( {1 - \left\lbrack \frac{{C.{y(T)}} - 1050}{p - 1050} \right\rbrack} \right)}} \right)}} & (18)\end{matrix}$

FIG. 24 shows a block diagram of a schematic process for determining agas concentration under consideration of the influences of the pressureand the temperature from the formed sensor signal sigX. In other words,FIG. 24 shows a schematic illustration for forming the sensor signalsigX from amplitudes and phases as well as the determination of a gasconcentration from sigX under the consideration of a pressure andtemperature influence (example).

Besides the calibration of the sensor signal to the concentration of aknown gas mixture, it is also possible to directly determine the thermaldiffusivity a of the gas mixture. In FIG. 25, the theoreticallycalculated thermal diffusivity is plotted with respect to the sensorsignal sigX. In other words, FIG. 25 shows the thermal diffusivity withrespect to the sensor signal sigX at a constant pressure and a constanttemperature in a mixture of carbon dioxide CO₂ in nitrogen N₂. Thethermal diffusivity 240 ₁ (red line) falls with an increase of the CO₂concentration 240 ₂ (green line).

Thus, a design and an evaluation of a thermal gas sensor for measuringphysical gas properties is described herein. With this invention, thefollowing is proposed (aspects are independent from each other and canbe used in combination):

-   -   sensor design based on two technology variations: a MEMS wire        sensor on a SOI substrate, and a thermopile sensor on a        thin-layer membrane    -   operation of the gas sensor: signal generation and signal        evaluation on an embedded system    -   evaluation algorithm for calibrating a gas mixture with a drift        correction with respect to a gas pressure and gas temperature

1.4 Market—Possible Application Areas (Optional)

In medical technology for respiration

In natural gas analysis—determination of the calorific value

There are various systems for patient ventilation on the market today.These are distinguished according to their use in the clinical and homecare sector (e.g. systems from Heinen+Löwenstein, Dräger and StephanMedizintechnik). The systems of these suppliers contain only in theirtop versions all the measuring equipment for determining pressure,respiratory flow, and respiratory gas analysis. To this end, severaldevices have to be combined, which mainly measure remotely from thepatient. From this, it may be derived that a cost-efficient measurementof a respiratory flow and CO₂ content close to the patient has not yetbeen implemented, and that the innovative content of the project istherefore confirmed with the development of a multi-sensor system withhybrid filters.

In our opinion, the successful development of the new MEMS-based gasmeasurement system represents a significant advance for the sensortechnology and respiratory care. The integration of both sensors (CO₂and flow) in one sensor system leads to a significant reduction of theinstallation space and the system weight (an essential criterion forintubated patients). Only the measuring point close to the patient,directly on the mask or tube—as close as possible to the airways—enablesa sufficiently accurate measurement to avoid influences from tubes,movements or sources of interference. In addition, the thermalmeasurement principle is expected to provide more accurate flowmeasurements and a rapid gas analysis.

In the following, further embodiments describing features andfunctionalities of the inventive gas sensor in other words areillustrated. These embodiments may be combined with the embodimentsdescribed above or may represent alternatives.

According to an embodiment, the gas sensor is a membrane sensor. Thethermal gas sensor based on the membrane and thermopile technology witha perforated membrane may be implemented to minimize the parasitic heattransport via the membrane or the suspensions of the structures in orderto obtain a higher gas-sensitive signal.

According to an embodiment, the inventive gas sensor may comprise anelectronic system, wherein the electronic system may comprise one orseveral of the following aspects, individually or in combination. Theelectronic system may comprise a DC sensor amplifier with an operatingpoint that is tracked via software. Furthermore, the electronic systemmay be implemented to perform a measurement of the phase position viathe internal timer structure of the micro controller (MSP430), wherein,e.g., the precise generation of the heater excitation signal via theanalog switch and the internal timer structure of the micro controller(MSP430) is used herein. In addition, the electronic system may beimplemented to perform a measurement of the phase position of the sensorsignals via a Schmitt-trigger that measures the sensor signals free ofthe DC-offset in the zero point crossing, since the signals are steepestthere and the phase noise is therefore minimized. Optionally, theelectronic system comprises a control of the heating power via a S1amplitude controller and/or a control of the timing of the sampling.

According to an embodiment, the gas sensor may have a calibration. Thecalibration may be configured to form a pseudo signal consisting of aphase and an amplitude, where the emphasis in the signal formation andthe equation may be placed on a pseudo signal.

It is to be noted that the embodiments according to the claims may besupplemented with all features, functionalities, and details describedherein (if this does not lead to any contradictions).

Features, functionalities, and details of the claims may also becombined with the embodiments described herein in order to obtainadditional embodiments.

It is to be noted that features and functionalities shown in individualembodiments or some of the embodiments may also be employed in otherembodiments if there are no significant technical reasons against this.

Furthermore, it is to be noted that partial functionalities of theembodiments described herein may be employed if there are no significanttechnical reasons against this.

Even though some aspects have been described within the context of adevice, it is understood that said aspects also represent a descriptionof the corresponding method, so that a block or a structural componentof a device is also to be understood as a corresponding method step oras a feature of a method step. By analogy therewith, aspects that havebeen described within the context of or as a method step also representa description of a corresponding block or detail or feature of acorresponding device. Some or all of the method steps may be performedwhile using a hardware device (or using a hardware device), such as amicroprocessor, a programmable computer or an electronic circuit. Insome embodiments, some or several of the most important method steps maybe performed by such a device.

Depending on specific implementation requirements, embodiments of theinvention may be implemented in hardware or in software. Implementationmay be effected while using a digital storage medium, for example afloppy disc, a DVD, a Blu-ray disc, a CD, a ROM, a PROM, an EPROM, anEEPROM or a FLASH memory, a hard disc or any other magnetic or opticalmemory which has electronically readable control signals stored thereonwhich may cooperate, or cooperate, with a programmable computer systemsuch that the respective method is performed. This is why the digitalstorage medium may be computer-readable.

Some embodiments in accordance with the invention thus comprise a datacarrier which comprises electronically readable control signals that arecapable of cooperating with a programmable computer system such that anyof the methods described herein is performed.

Generally, embodiments of the present invention may be implemented as acomputer program product having a program code, the program code beingeffective to perform any of the methods when the computer programproduct runs on a computer.

The program code may also be stored on a machine-readable carrier, forexample.

Other embodiments include the computer program for performing any of themethods described herein, said computer program being stored on amachine-readable carrier.

In other words, an embodiment of the inventive method thus is a computerprogram which has a program code for performing any of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods thus is a data carrier (ora digital storage medium or a computer-readable medium) on which thecomputer program for performing any of the methods described herein isrecorded. The data carrier, the digital storage medium, or the recordedmedium are typically tangible, or non-volatile.

A further embodiment of the inventive method thus is a data stream or asequence of signals representing the computer program for performing anyof the methods described herein.

The data stream or the sequence of signals may be configured, forexample, to be transmitted via a data communication link, for examplevia the internet.

A further embodiment includes a processing unit, for example a computeror a programmable logic device, configured or adapted to perform any ofthe methods described herein.

A further embodiment includes a computer on which the computer programfor performing any of the methods described herein is installed.

A further embodiment in accordance with the invention includes a deviceor a system configured to transmit a computer program for performing atleast one of the methods described herein to a receiver. Thetransmission may be electronic or optical, for example. The receiver maybe a computer, a mobile device, a memory device or a similar device, forexample. The device or the system may include a file server fortransmitting the computer program to the receiver, for example.

In some embodiments, a programmable logic device (for example afield-programmable gate array, an FPGA) may be used for performing someor all of the functionalities of the methods described herein. In someembodiments, a field-programmable gate array may cooperate with amicroprocessor to perform any of the methods described herein.Generally, the methods are performed, in some embodiments, by anyhardware device. Said hardware device may be any universally applicablehardware such as a computer processor (CPU), or may be a hardwarespecific to the method, such as an ASIC.

For example, the apparatuses described herein may be implemented using ahardware device, or using a computer, or using a combination of ahardware device and a computer.

The apparatuses described herein, or any components of the apparatusesdescribed herein, may at least be partially implement in hardware and/orsoftware (computer program).

For example, the methods described herein may be implemented using ahardware device, or using a computer, or using a combination of ahardware device and a computer.

The methods described herein, or any components of the methods describedherein, may at least be partially implement by performed and/or software(computer program).

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutationsand equivalents as fall within the true spirit and scope of the presentinvention.

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1. Gas sensor, comprising: a membrane; a heating element arranged on themembrane between a first discontinuation area of the membrane and asecond discontinuation area of the membrane, wherein the firstdiscontinuation area of the membrane comprises at least onediscontinuation of the membrane, and wherein the second discontinuationarea of the membrane comprises at least one discontinuation of themembrane, a first temperature sensor structure arranged at leastpartially on the membrane on a side of the first discontinuation area ofthe membrane opposite to the heating element; and a second temperaturesensor structure arranged at least partially on the membrane on a sideof the second discontinuation area of the membrane opposite to theheating element.
 2. Gas sensor according to claim 1, wherein themembrane is spread out by a frame made of a carrier material implementedsuch that the coefficient of temperature expansion of the membranedeviates from the coefficient of temperature expansion of the carriermaterial.
 3. Gas sensor according to claim 1, wherein the firsttemperature sensor structure is a first thermal element structure havinga hot end arranged on the membrane on a side of the firstdiscontinuation area of the membrane opposite to the heating element;wherein the second temperature sensor structure is a second thermalelement structure having a hot end arranged on the membrane on a side ofthe second discontinuation area of the membrane opposite to the heatingelement.
 4. Gas sensor according to claim 2, wherein cold ends of thefirst thermal element structure and cold ends of the second thermalelement structure are arranged on the carrier material.
 5. Gas sensoraccording to claim 3, wherein the hot end of the first thermal elementstructure reaches up to an edge of the first discontinuation area of themembrane, and wherein the hot end of the second thermal elementstructure reaches up to an edge of the second discontinuation area ofthe membrane.
 6. Gas sensor according to claim 1, wherein the firstdiscontinuation area of the membrane comprises a continuousdiscontinuation whose longitudinal expansion is large enough that itfully covers the area between the first temperature sensor structure andthe heating element, and wherein the second discontinuation area of themembrane comprises a continuous discontinuation whose longitudinalexpansion is large enough that it fully covers the area between thesecond temperature sensor structure and the heating element.
 7. Gassensor according to claim 1, wherein a lateral expansion of thediscontinuation of the first discontinuation area differs from a lateralexpansion of the discontinuation of the second discontinuation area. 8.Gas sensor according to claim 1, wherein the first temperature sensorstructure comprises a different distance to the heating element than thesecond temperature sensor structure.
 9. Gas sensor according to claim 1,wherein the first temperature sensor structure comprises a same distanceto the heating element as the second temperature sensor structure. 10.Gas sensor according to claim 1, wherein the first discontinuation areaand the second discontinuation area comprise several discontinuationsarranged such that a grid structure in which the discontinuations arearranged in rows in parallel to the heating element and the rows arearranged offset to each other is created.
 11. Gas sensor according toclaim 1, wherein the first discontinuation area and the seconddiscontinuation area comprise several discontinuations arranged suchthat a grid structure in which a path of a heat conduction through themembrane is longer than a direct path is created.
 12. Gas sensoraccording to claim 1, wherein the discontinuations in the firstdiscontinuation area and in the second discontinuation area arerectangular cutouts with rounded edges.
 13. Gas sensor according toclaim 1, wherein the discontinuations in the first discontinuation areaand in the second discontinuation area are at least three times as longas they are wide.
 14. Gas sensor according to claim 1, wherein adistance between the discontinuations in the first discontinuation areaand a distance between the discontinuations in the seconddiscontinuation area correspond to the smallest realizable structuralwidth resulting in a mechanically durable grid structure.
 15. Gas sensoraccording to claim 1, wherein the heating element, the first thermalelement structure, and/or the second thermal element structure arepassivated with a protective layer.
 16. Method for operating a gassensor, comprising: heating a heating element; conducting heat via a gasmixture, wherein more heat is conducted from the heating element to atemperature sensor structure via the gas mixture surrounding the gassensor than via a membrane; and detecting a heating transfer by means ofthe hot ends of a temperature sensor structure.
 17. Method according toclaim 16, wherein the method comprises determining a gas concentrationand/or a gas composition and/or a gas flow based on the detection of theheat transfer.
 18. Gas sensor, comprising: a membrane; a heating elementarranged on the membrane between a first discontinuation area of themembrane and a second discontinuation area of the membrane, wherein thefirst discontinuation area of the membrane comprises at least onediscontinuation of the membrane, and wherein the second discontinuationarea of the membrane comprises at least one discontinuation of themembrane, a first thermal element structure having a hot end arranged onthe membrane on a side of the first discontinuation area of the membraneopposite to the heating element; and a second thermal element structurehaving a hot end arranged on the membrane on a side of the seconddiscontinuation area of the membrane opposite to the heating element.